diff --git a/CHANGELOG.md b/CHANGELOG.md
index 667fc019..e21ceed3 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,8 @@
 All notable changes to this project will be documented in this file. This change log follows the conventions of [keepachangelog.com](http://keepachangelog.com/).
 
 ## [Unreleased]
+### Added
+- Add `planck.io/tty?` ([#911](https://github.com/planck-repl/planck/issues/911))
 
 ## [2.22.0] - 2019-04-06
 ### Added
diff --git a/int-test/expected/PLANCK-OUT.txt b/int-test/expected/PLANCK-OUT.txt
index 0937b906..3b75a84b 100644
--- a/int-test/expected/PLANCK-OUT.txt
+++ b/int-test/expected/PLANCK-OUT.txt
@@ -611,3 +611,13 @@ Execution error (ReferenceError) at (<cljs repl>:1).
 Can't find variable: a
 Auto-load user.cljs
 3
+stdio TTY is detected (in a faketty)
+[[*in* true] [*out* true] [*err* true]]
+stdin non-TTY is detected (in a pipeline)
+[*in* false]
+stdout non-TTY is detected (in a pipeline)
+[*out* false]
+stderr non-TTY is detected (redirected to /dev/null)
+[*err* false]
+*err* TTY is detected when rebound to *out* even when stderr is redirected to /dev/null
+[[*out* true] [*err* true]]
diff --git a/int-test/script/gen-actual b/int-test/script/gen-actual
index fa925994..ed7e9b84 100755
--- a/int-test/script/gen-actual
+++ b/int-test/script/gen-actual
@@ -795,3 +795,35 @@ echo "(def x 3)" > /tmp/PLANCK_SRC/user.cljs
 $PLANCK -c /tmp/PLANCK_SRC <<REPL_INPUT
 x
 REPL_INPUT
+
+faketty () {
+    if [[ "$OSTYPE" == "darwin"* ]]; then
+        script -qt 0 /dev/null $@
+    else
+        script -qfec "$(printf "%q " "$@")" /dev/null
+    fi
+}
+echo "stdio TTY is detected (in a faketty)"
+faketty $PLANCK -i $SRC/test_tty/stdio.cljs
+echo
+
+echo "stdin non-TTY is detected (in a pipeline)"
+echo "" | $PLANCK -i $SRC/test_tty/stdin.cljs
+echo
+
+echo "stdout non-TTY is detected (in a pipeline)"
+$PLANCK -i $SRC/test_tty/stdout.cljs | cat
+echo
+
+echo "stderr non-TTY is detected (redirected to /dev/null)"
+$PLANCK -i $SRC/test_tty/stderr.cljs 2>/dev/null
+echo
+
+echo "*err* TTY is detected when rebound to *out* even when stderr is redirected to /dev/null"
+cat <<EOF >/tmp/PLANCK_TTY_REBINDING_TEST
+#!/bin/sh
+$PLANCK -i $SRC/test_tty/rebinding_err_out.cljs 2>/dev/null
+EOF
+chmod +x /tmp/PLANCK_TTY_REBINDING_TEST
+faketty /tmp/PLANCK_TTY_REBINDING_TEST
+echo
diff --git a/int-test/src/test_tty/rebinding_err_out.cljs b/int-test/src/test_tty/rebinding_err_out.cljs
new file mode 100644
index 00000000..77811327
--- /dev/null
+++ b/int-test/src/test_tty/rebinding_err_out.cljs
@@ -0,0 +1,5 @@
+(require 'planck.core)
+(require 'planck.io)
+(binding [planck.core/*err* cljs.core/*out*]
+  (pr [['*out* (planck.io/tty? cljs.core/*out*)]
+       ['*err* (planck.io/tty? planck.core/*err*)]]))
diff --git a/int-test/src/test_tty/stderr.cljs b/int-test/src/test_tty/stderr.cljs
new file mode 100644
index 00000000..2d38be17
--- /dev/null
+++ b/int-test/src/test_tty/stderr.cljs
@@ -0,0 +1,3 @@
+(require 'planck.core)
+(require 'planck.io)
+(pr ['*err* (planck.io/tty? planck.core/*err*)])
diff --git a/int-test/src/test_tty/stdin.cljs b/int-test/src/test_tty/stdin.cljs
new file mode 100644
index 00000000..0e03db6d
--- /dev/null
+++ b/int-test/src/test_tty/stdin.cljs
@@ -0,0 +1,3 @@
+(require 'planck.core)
+(require 'planck.io)
+(pr ['*in* (planck.io/tty? planck.core/*in*)])
diff --git a/int-test/src/test_tty/stdio.cljs b/int-test/src/test_tty/stdio.cljs
new file mode 100644
index 00000000..e0dec5d1
--- /dev/null
+++ b/int-test/src/test_tty/stdio.cljs
@@ -0,0 +1,5 @@
+(require 'planck.core)
+(require 'planck.io)
+(pr [['*in*  (planck.io/tty? planck.core/*in*)]
+     ['*out* (planck.io/tty? cljs.core/*out*)]
+     ['*err* (planck.io/tty? planck.core/*err*)]])
diff --git a/int-test/src/test_tty/stdout.cljs b/int-test/src/test_tty/stdout.cljs
new file mode 100644
index 00000000..bcf74039
--- /dev/null
+++ b/int-test/src/test_tty/stdout.cljs
@@ -0,0 +1,3 @@
+(require 'planck.core)
+(require 'planck.io)
+(pr ['*out* (planck.io/tty? cljs.core/*out*)])
diff --git a/planck-c/engine.c b/planck-c/engine.c
index ca9d1747..dcd55c44 100644
--- a/planck-c/engine.c
+++ b/planck-c/engine.c
@@ -523,6 +523,8 @@ void *do_engine_init(void *data) {
 
     register_global_function(ctx, "PLANCK_GETENV", function_getenv);
 
+    register_global_function(ctx, "PLANCK_ISATTY", function_isatty);
+
     display_launch_timing("register fns");
 
     // Monkey patch cljs.core/system-time to use Planck's high-res timer
diff --git a/planck-c/functions.c b/planck-c/functions.c
index 0ea08c5a..21b17e04 100644
--- a/planck-c/functions.c
+++ b/planck-c/functions.c
@@ -1522,3 +1522,18 @@ JSValueRef function_getenv(JSContextRef ctx, JSObjectRef function, JSObjectRef t
   }
   return JSValueMakeNull(ctx);
 }
+
+JSValueRef function_isatty(JSContextRef ctx, JSObjectRef function, JSObjectRef thisObject,
+                           size_t argc, const JSValueRef args[], JSValueRef *exception) {
+  if (argc == 1 && JSValueGetType(ctx, args[0]) == kJSTypeNumber) {
+    int fd = (int) JSValueToNumber(ctx, args[0], NULL);
+    if (fd >= 0) {
+      errno = 0;
+      bool result = isatty(fd);
+      if (errno != EBADF) {
+        return JSValueMakeBoolean(ctx, result);
+      }
+    }
+  }
+  return JSValueMakeNull(ctx);
+}
diff --git a/planck-c/functions.h b/planck-c/functions.h
index ffef4d6c..17488797 100644
--- a/planck-c/functions.h
+++ b/planck-c/functions.h
@@ -156,3 +156,6 @@ JSValueRef function_signal_task_complete(JSContextRef ctx, JSObjectRef function,
 
 JSValueRef function_getenv(JSContextRef ctx, JSObjectRef function, JSObjectRef thisObject,
                            size_t argc, const JSValueRef args[], JSValueRef *exception);
+
+JSValueRef function_isatty(JSContextRef ctx, JSObjectRef function, JSObjectRef thisObject,
+                           size_t argc, const JSValueRef args[], JSValueRef *exception);
diff --git a/planck-cljs/src/planck/io.cljs b/planck-cljs/src/planck/io.cljs
index 688678c1..64509286 100644
--- a/planck-cljs/src/planck/io.cljs
+++ b/planck-cljs/src/planck/io.cljs
@@ -552,6 +552,29 @@
   :args (s/cat :input any? :output any? :opts (s/* any?))
   :ret nil?)
 
+(def ^:private stdio->fd
+  {planck.core/*in*  0
+   cljs.core/*out*   1
+   planck.core/*err* 2})
+
+(defn ^boolean tty?
+  "Returns true if x is a file descriptor associated with a terminal,
+  or x is either a Reader/Writer among *in*, *out*, or *err* which is
+  associated with a terminal.
+
+  Returns false if x is a file descriptor, *in*, *out*, or *err* and
+  not associated with a terminal, or an invalid file descriptor."
+  [x]
+  (-> (if-let [fd (stdio->fd x)] fd x)
+      js/PLANCK_ISATTY
+      boolean))
+
+(s/fdef tty?
+  :args (s/cat :x (s/or :fd-num (s/and integer? (complement neg?))
+                        :reader #(implements? planck.core/IReader %)
+                        :writer #(implements? planck.core/IWriter %)))
+  :ret boolean?)
+
 ;; These have been moved
 (def ^:deprecated read-line planck.core/read-line)
 (def ^:deprecated slurp planck.core/slurp)
diff --git a/planck-cljs/test/planck/io_test.cljs b/planck-cljs/test/planck/io_test.cljs
index e2ae9daa..92b86355 100644
--- a/planck-cljs/test/planck/io_test.cljs
+++ b/planck-cljs/test/planck/io_test.cljs
@@ -71,7 +71,17 @@
       (is (= true  (planck.io/exists? hidden-directory)))
       (is (= true  (planck.io/hidden-file? hidden-directory)))
       (is (= false (planck.io/regular-file? hidden-directory)))
-      (is (= false (planck.io/symbolic-link? hidden-directory))))))
+      (is (= false (planck.io/symbolic-link? hidden-directory)))
+      (is (boolean? (planck.io/tty? 0)))
+      (is (boolean? (planck.io/tty? 1)))
+      (is (boolean? (planck.io/tty? 2)))
+      (is (boolean? (planck.io/tty? planck.core/*in*)))
+      (is (boolean? (planck.io/tty? *out*)))
+      (is (boolean? (planck.io/tty? planck.core/*err*)))
+      (is (= false (planck.io/tty? nil)))
+      (is (= false (planck.io/tty? -1)))
+      (is (= false (planck.io/tty? 99999999999999999)))
+      (is (= false (planck.io/tty? (planck.io/reader regular-file)))))))
 
 (deftest coercions
   (testing "as-file coerceions"
diff --git a/site/src/planck-io.md b/site/src/planck-io.md
index 73d983d7..1851680a 100644
--- a/site/src/planck-io.md
+++ b/site/src/planck-io.md
@@ -38,6 +38,7 @@ _Vars_
 [regular-file?](#regular-file?)<br/>
 [resource](#resource)<br/>
 [symbolic-link?](#symbolic-link?)<br/>
+[tty?](#tty?)<br/>
 [writer](#writer)<br/>
    
 ## Protocols
@@ -299,6 +300,15 @@ Spec<br/>
  _args_: `(cat :f (or :string string? :file file?))`<br/>
  _ret_: `boolean?`<br/>
 
+### <a name="tty?"></a>tty?
+`([x])`
+
+Checks if `x` is a file descriptor associated with a terminal. `x` may be a file descriptor number or one of `planck.core/*in*`, `cljs.core/*out*` or `planck.core/*err*`.
+
+Spec<br/>
+_x_: `(or :fd-num (and integer? (complement neg?)) :reader #(implements? planck.core/IReader %) :writer #(implements? planck.core/IWriter %))`<br/>
+_ret_: `boolean?`<br/>
+
 ### <a name="writer"></a>writer
 `([x & opts])`