diff --git a/compiler/rustc_codegen_gcc/src/builder.rs b/compiler/rustc_codegen_gcc/src/builder.rs
index f5cda81f6ab86..3cfc308c54638 100644
--- a/compiler/rustc_codegen_gcc/src/builder.rs
+++ b/compiler/rustc_codegen_gcc/src/builder.rs
@@ -31,7 +31,7 @@ use rustc_span::Span;
 use rustc_target::abi::{
     self, call::FnAbi, Align, HasDataLayout, Size, TargetDataLayout, WrappingRange,
 };
-use rustc_target::spec::{HasTargetSpec, Target};
+use rustc_target::spec::{HasTargetSpec, HasWasmCAbiOpt, Target, WasmCAbi};
 
 use crate::common::{type_is_pointer, SignType, TypeReflection};
 use crate::context::CodegenCx;
@@ -2349,6 +2349,12 @@ impl<'tcx> HasTargetSpec for Builder<'_, '_, 'tcx> {
     }
 }
 
+impl<'tcx> HasWasmCAbiOpt for Builder<'_, '_, 'tcx> {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi {
+        self.cx.wasm_c_abi_opt()
+    }
+}
+
 pub trait ToGccComp {
     fn to_gcc_comparison(&self) -> ComparisonOp;
 }
diff --git a/compiler/rustc_codegen_gcc/src/context.rs b/compiler/rustc_codegen_gcc/src/context.rs
index bc3d62f2679d9..99c079f40e03b 100644
--- a/compiler/rustc_codegen_gcc/src/context.rs
+++ b/compiler/rustc_codegen_gcc/src/context.rs
@@ -20,7 +20,7 @@ use rustc_span::{source_map::respan, Span};
 use rustc_target::abi::{
     call::FnAbi, HasDataLayout, PointeeInfo, Size, TargetDataLayout, VariantIdx,
 };
-use rustc_target::spec::{HasTargetSpec, Target, TlsModel};
+use rustc_target::spec::{HasTargetSpec, HasWasmCAbiOpt, Target, TlsModel, WasmCAbi};
 
 use crate::callee::get_fn;
 use crate::common::SignType;
@@ -555,6 +555,12 @@ impl<'gcc, 'tcx> HasTargetSpec for CodegenCx<'gcc, 'tcx> {
     }
 }
 
+impl<'gcc, 'tcx> HasWasmCAbiOpt for CodegenCx<'gcc, 'tcx> {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi {
+        self.tcx.sess.opts.unstable_opts.wasm_c_abi
+    }
+}
+
 impl<'gcc, 'tcx> LayoutOfHelpers<'tcx> for CodegenCx<'gcc, 'tcx> {
     type LayoutOfResult = TyAndLayout<'tcx>;
 
diff --git a/compiler/rustc_interface/src/tests.rs b/compiler/rustc_interface/src/tests.rs
index 4f15d1c1d3a0b..758118cb38beb 100644
--- a/compiler/rustc_interface/src/tests.rs
+++ b/compiler/rustc_interface/src/tests.rs
@@ -17,7 +17,9 @@ use rustc_session::{build_session, getopts, CompilerIO, EarlyDiagCtxt, Session};
 use rustc_span::edition::{Edition, DEFAULT_EDITION};
 use rustc_span::symbol::sym;
 use rustc_span::{FileName, SourceFileHashAlgorithm};
-use rustc_target::spec::{CodeModel, LinkerFlavorCli, MergeFunctions, PanicStrategy, RelocModel};
+use rustc_target::spec::{
+    CodeModel, LinkerFlavorCli, MergeFunctions, PanicStrategy, RelocModel, WasmCAbi,
+};
 use rustc_target::spec::{RelroLevel, SanitizerSet, SplitDebuginfo, StackProtector, TlsModel};
 use std::collections::{BTreeMap, BTreeSet};
 use std::num::NonZero;
@@ -836,6 +838,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(verify_llvm_ir, true);
     tracked!(virtual_function_elimination, true);
     tracked!(wasi_exec_model, Some(WasiExecModel::Reactor));
+    tracked!(wasm_c_abi, WasmCAbi::Spec);
     // tidy-alphabetical-end
 
     macro_rules! tracked_no_crate_hash {
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index b1bfd2f1105a8..e7ef7e381c2f8 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -15,7 +15,9 @@ use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{ErrorGuaranteed, Span, DUMMY_SP};
 use rustc_target::abi::call::FnAbi;
 use rustc_target::abi::*;
-use rustc_target::spec::{abi::Abi as SpecAbi, HasTargetSpec, PanicStrategy, Target};
+use rustc_target::spec::{
+    abi::Abi as SpecAbi, HasTargetSpec, HasWasmCAbiOpt, PanicStrategy, Target, WasmCAbi,
+};
 
 use std::borrow::Cow;
 use std::cmp;
@@ -539,6 +541,12 @@ impl<'tcx> HasTargetSpec for TyCtxt<'tcx> {
     }
 }
 
+impl<'tcx> HasWasmCAbiOpt for TyCtxt<'tcx> {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi {
+        self.sess.opts.unstable_opts.wasm_c_abi
+    }
+}
+
 impl<'tcx> HasTyCtxt<'tcx> for TyCtxt<'tcx> {
     #[inline]
     fn tcx(&self) -> TyCtxt<'tcx> {
@@ -584,6 +592,12 @@ impl<'tcx, T: HasTargetSpec> HasTargetSpec for LayoutCx<'tcx, T> {
     }
 }
 
+impl<'tcx, T: HasWasmCAbiOpt> HasWasmCAbiOpt for LayoutCx<'tcx, T> {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi {
+        self.tcx.wasm_c_abi_opt()
+    }
+}
+
 impl<'tcx, T: HasTyCtxt<'tcx>> HasTyCtxt<'tcx> for LayoutCx<'tcx, T> {
     fn tcx(&self) -> TyCtxt<'tcx> {
         self.tcx.tcx()
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index 61a220428b03b..2213a626d4fef 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -3227,7 +3227,7 @@ pub(crate) mod dep_tracking {
     use rustc_feature::UnstableFeatures;
     use rustc_span::edition::Edition;
     use rustc_span::RealFileName;
-    use rustc_target::spec::{CodeModel, MergeFunctions, PanicStrategy, RelocModel};
+    use rustc_target::spec::{CodeModel, MergeFunctions, PanicStrategy, RelocModel, WasmCAbi};
     use rustc_target::spec::{
         RelroLevel, SanitizerSet, SplitDebuginfo, StackProtector, TargetTriple, TlsModel,
     };
@@ -3324,6 +3324,7 @@ pub(crate) mod dep_tracking {
         Polonius,
         InliningThreshold,
         FunctionReturn,
+        WasmCAbi,
     );
 
     impl<T1, T2> DepTrackingHash for (T1, T2)
diff --git a/compiler/rustc_session/src/options.rs b/compiler/rustc_session/src/options.rs
index 93bef82e4ba15..75f3130a8bd57 100644
--- a/compiler/rustc_session/src/options.rs
+++ b/compiler/rustc_session/src/options.rs
@@ -8,7 +8,9 @@ use rustc_data_structures::profiling::TimePassesFormat;
 use rustc_data_structures::stable_hasher::Hash64;
 use rustc_errors::ColorConfig;
 use rustc_errors::{LanguageIdentifier, TerminalUrl};
-use rustc_target::spec::{CodeModel, LinkerFlavorCli, MergeFunctions, PanicStrategy, SanitizerSet};
+use rustc_target::spec::{
+    CodeModel, LinkerFlavorCli, MergeFunctions, PanicStrategy, SanitizerSet, WasmCAbi,
+};
 use rustc_target::spec::{
     RelocModel, RelroLevel, SplitDebuginfo, StackProtector, TargetTriple, TlsModel,
 };
@@ -437,6 +439,7 @@ mod desc {
         "either a boolean (`yes`, `no`, `on`, `off`, etc), or a non-negative number";
     pub const parse_llvm_module_flag: &str = "<key>:<type>:<value>:<behavior>. Type must currently be `u32`. Behavior should be one of (`error`, `warning`, `require`, `override`, `append`, `appendunique`, `max`, `min`)";
     pub const parse_function_return: &str = "`keep` or `thunk-extern`";
+    pub const parse_wasm_c_abi: &str = "`legacy` or `spec`";
 }
 
 mod parse {
@@ -1402,6 +1405,15 @@ mod parse {
         }
         true
     }
+
+    pub(crate) fn parse_wasm_c_abi(slot: &mut WasmCAbi, v: Option<&str>) -> bool {
+        match v {
+            Some("spec") => *slot = WasmCAbi::Spec,
+            Some("legacy") => *slot = WasmCAbi::Legacy,
+            _ => return false,
+        }
+        true
+    }
 }
 
 options! {
@@ -2021,6 +2033,8 @@ written to standard error output)"),
         Requires `-Clto[=[fat,yes]]`"),
     wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
         "whether to build a wasi command or reactor"),
+    wasm_c_abi: WasmCAbi = (WasmCAbi::Legacy, parse_wasm_c_abi, [TRACKED],
+        "use spec-compliant C ABI for `wasm32-unknown-unknown` (default: legacy)"),
     write_long_types_to_disk: bool = (true, parse_bool, [UNTRACKED],
         "whether long type names should be written to files instead of being printed in errors"),
     // tidy-alphabetical-end
diff --git a/compiler/rustc_target/src/abi/call/mod.rs b/compiler/rustc_target/src/abi/call/mod.rs
index f4967b94e930a..930edfe1f0df3 100644
--- a/compiler/rustc_target/src/abi/call/mod.rs
+++ b/compiler/rustc_target/src/abi/call/mod.rs
@@ -1,6 +1,6 @@
 use crate::abi::{self, Abi, Align, FieldsShape, Size};
 use crate::abi::{HasDataLayout, TyAbiInterface, TyAndLayout};
-use crate::spec::{self, HasTargetSpec};
+use crate::spec::{self, HasTargetSpec, HasWasmCAbiOpt};
 use rustc_span::Symbol;
 use std::fmt;
 use std::str::FromStr;
@@ -782,7 +782,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
     ) -> Result<(), AdjustForForeignAbiError>
     where
         Ty: TyAbiInterface<'a, C> + Copy,
-        C: HasDataLayout + HasTargetSpec,
+        C: HasDataLayout + HasTargetSpec + HasWasmCAbiOpt,
     {
         if abi == spec::abi::Abi::X86Interrupt {
             if let Some(arg) = self.args.first_mut() {
@@ -839,7 +839,9 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "sparc" => sparc::compute_abi_info(cx, self),
             "sparc64" => sparc64::compute_abi_info(cx, self),
             "nvptx64" => {
-                if cx.target_spec().adjust_abi(abi, self.c_variadic) == spec::abi::Abi::PtxKernel {
+                if cx.target_spec().adjust_abi(cx, abi, self.c_variadic)
+                    == spec::abi::Abi::PtxKernel
+                {
                     nvptx64::compute_ptx_kernel_abi_info(cx, self)
                 } else {
                     nvptx64::compute_abi_info(self)
@@ -848,7 +850,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "hexagon" => hexagon::compute_abi_info(self),
             "riscv32" | "riscv64" => riscv::compute_abi_info(cx, self),
             "wasm32" | "wasm64" => {
-                if cx.target_spec().adjust_abi(abi, self.c_variadic) == spec::abi::Abi::Wasm {
+                if cx.target_spec().adjust_abi(cx, abi, self.c_variadic) == spec::abi::Abi::Wasm {
                     wasm::compute_wasm_abi_info(self)
                 } else {
                     wasm::compute_c_abi_info(cx, self)
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 3b6e15fad4661..c42e818167c3b 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -1807,6 +1807,19 @@ impl HasTargetSpec for Target {
     }
 }
 
+/// Which C ABI to use for `wasm32-unknown-unknown`.
+#[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
+pub enum WasmCAbi {
+    /// Spec-compliant C ABI.
+    Spec,
+    /// Legacy ABI. Which is non-spec-compliant.
+    Legacy,
+}
+
+pub trait HasWasmCAbiOpt {
+    fn wasm_c_abi_opt(&self) -> WasmCAbi;
+}
+
 type StaticCow<T> = Cow<'static, T>;
 
 /// Optional aspects of a target specification.
@@ -2417,9 +2430,21 @@ impl DerefMut for Target {
 
 impl Target {
     /// Given a function ABI, turn it into the correct ABI for this target.
-    pub fn adjust_abi(&self, abi: Abi, c_variadic: bool) -> Abi {
+    pub fn adjust_abi<C>(&self, cx: &C, abi: Abi, c_variadic: bool) -> Abi
+    where
+        C: HasWasmCAbiOpt,
+    {
         match abi {
-            Abi::C { .. } => self.default_adjusted_cabi.unwrap_or(abi),
+            Abi::C { .. } => {
+                if self.arch == "wasm32"
+                    && self.os == "unknown"
+                    && cx.wasm_c_abi_opt() == WasmCAbi::Spec
+                {
+                    abi
+                } else {
+                    self.default_adjusted_cabi.unwrap_or(abi)
+                }
+            }
 
             // On Windows, `extern "system"` behaves like msvc's `__stdcall`.
             // `__stdcall` only applies on x86 and on non-variadic functions:
diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs
index a5328baadb5fc..71bcf898ceab2 100644
--- a/compiler/rustc_ty_utils/src/abi.rs
+++ b/compiler/rustc_ty_utils/src/abi.rs
@@ -311,7 +311,7 @@ fn fn_sig_for_fn_abi<'tcx>(
 #[inline]
 fn conv_from_spec_abi(tcx: TyCtxt<'_>, abi: SpecAbi, c_variadic: bool) -> Conv {
     use rustc_target::spec::abi::Abi::*;
-    match tcx.sess.target.adjust_abi(abi, c_variadic) {
+    match tcx.sess.target.adjust_abi(&tcx, abi, c_variadic) {
         RustIntrinsic | Rust | RustCall => Conv::Rust,
 
         // This is intentionally not using `Conv::Cold`, as that has to preserve
diff --git a/src/doc/unstable-book/src/compiler-flags/wasm-c-abi.md b/src/doc/unstable-book/src/compiler-flags/wasm-c-abi.md
new file mode 100644
index 0000000000000..138a98dbe3dcd
--- /dev/null
+++ b/src/doc/unstable-book/src/compiler-flags/wasm-c-abi.md
@@ -0,0 +1,10 @@
+# `wasm-c-abi`
+
+This option controls whether Rust uses the spec-compliant C ABI when compiling
+for the `wasm32-unknown-unknown` target.
+
+This makes it possible to be ABI-compatible with all other spec-compliant Wasm
+like Rusts `wasm32-wasi`.
+
+This compiler flag is perma-unstable, as it will be enabled by default in the
+future with no option to fall back to the old non-spec-compliant ABI.