diff --git a/stdlib/LinearAlgebra/src/diagonal.jl b/stdlib/LinearAlgebra/src/diagonal.jl
index ed01cadd9f92f..95406701e3048 100644
--- a/stdlib/LinearAlgebra/src/diagonal.jl
+++ b/stdlib/LinearAlgebra/src/diagonal.jl
@@ -154,9 +154,9 @@ end
 (*)(D::Diagonal, B::AbstractTriangular) = lmul!(D, copy(B))
 
 (*)(A::AbstractMatrix, D::Diagonal) =
-    mul!(similar(A, promote_op(*, eltype(A), eltype(D.diag)), size(A)), A, D)
+    rmul!(copyto!(similar(A, promote_op(*, eltype(A), eltype(D.diag)), size(A)), A), D)
 (*)(D::Diagonal, A::AbstractMatrix) =
-    mul!(similar(A, promote_op(*, eltype(A), eltype(D.diag)), size(A)), D, A)
+    lmul!(D, copyto!(similar(A, promote_op(*, eltype(A), eltype(D.diag)), size(A)), A))
 
 function rmul!(A::AbstractMatrix, D::Diagonal)
     A .= A .* transpose(D.diag)
@@ -271,14 +271,6 @@ mul!(out::AbstractMatrix, A::Diagonal, in::AbstractMatrix) = out .= A.diag .* in
 mul!(out::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, in::AbstractMatrix) = out .= adjoint.(A.parent.diag) .* in
 mul!(out::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, in::AbstractMatrix) = out .= transpose.(A.parent.diag) .* in
 
-mul!(C::AbstractMatrix, A::Diagonal, B::Adjoint{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
-mul!(C::AbstractMatrix, A::Diagonal, B::Transpose{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
-mul!(C::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, B::Adjoint{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
-mul!(C::AbstractMatrix, A::Adjoint{<:Any,<:Diagonal}, B::Transpose{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
-mul!(C::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, B::Adjoint{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
-mul!(C::AbstractMatrix, A::Transpose{<:Any,<:Diagonal}, B::Transpose{<:Any,<:AbstractVecOrMat}) = mul!(C, A, copy(B))
-
-
 # ambiguities with Symmetric/Hermitian
 # RealHermSymComplex[Sym]/[Herm] only include Number; invariant to [c]transpose
 *(A::Diagonal, transB::Transpose{<:Any,<:RealHermSymComplexSym}) = A * transB.parent
@@ -478,8 +470,10 @@ function svdfact(D::Diagonal)
 end
 
 # dismabiguation methods: * of Diagonal and Adj/Trans AbsVec
-*(A::Diagonal, B::Adjoint{<:Any,<:AbstractVector}) = A * copy(B)
-*(A::Diagonal, B::Transpose{<:Any,<:AbstractVector}) = A * copy(B)
-*(A::Adjoint{<:Any,<:AbstractVector}, B::Diagonal) = copy(A) * B
-*(A::Transpose{<:Any,<:AbstractVector}, B::Diagonal) = copy(A) * B
+*(x::Adjoint{<:Any,<:AbstractVector}, D::Diagonal) = Adjoint(map((t,s) -> t'*s, D.diag, parent(x)))
+*(x::Adjoint{<:Any,<:AbstractVector}, D::Diagonal, y::AbstractVector) =
+    mapreduce(t -> t[1]*t[2]*t[3], +, zip(x, D.diag, y))
+*(x::Transpose{<:Any,<:AbstractVector}, D::Diagonal) = Transpose(map(*, D.diag, parent(x)))
+*(x::Transpose{<:Any,<:AbstractVector}, D::Diagonal, y::AbstractVector) =
+    mapreduce(t -> t[1]*t[2]*t[3], +, zip(x, D.diag, y))
 # TODO: these methods will yield row matrices, rather than adjoint/transpose vectors
diff --git a/stdlib/LinearAlgebra/src/matmul.jl b/stdlib/LinearAlgebra/src/matmul.jl
index 9619a5b630987..1c5bcbf027322 100644
--- a/stdlib/LinearAlgebra/src/matmul.jl
+++ b/stdlib/LinearAlgebra/src/matmul.jl
@@ -51,12 +51,8 @@ end
 # these will throw a DimensionMismatch unless B has 1 row (or 1 col for transposed case):
 *(a::AbstractVector, transB::Transpose{<:Any,<:AbstractMatrix}) =
     (B = transB.parent; *(reshape(a,length(a),1), transpose(B)))
-*(A::AbstractMatrix, transb::Transpose{<:Any,<:AbstractVector}) =
-    (b = transb.parent; *(A, transpose(reshape(b,length(b),1))))
 *(a::AbstractVector, adjB::Adjoint{<:Any,<:AbstractMatrix}) =
     (B = adjB.parent; *(reshape(a,length(a),1), adjoint(B)))
-*(A::AbstractMatrix, adjb::Adjoint{<:Any,<:AbstractVector}) =
-    (b = adjb.parent; *(A, adjoint(reshape(b,length(b),1))))
 (*)(a::AbstractVector, B::AbstractMatrix) = reshape(a,length(a),1)*B
 
 mul!(y::StridedVector{T}, A::StridedVecOrMat{T}, x::StridedVector{T}) where {T<:BlasFloat} = gemv!(y, 'N', A, x)
diff --git a/stdlib/LinearAlgebra/test/diagonal.jl b/stdlib/LinearAlgebra/test/diagonal.jl
index 540111c0012a2..06a5d78ba2c84 100644
--- a/stdlib/LinearAlgebra/test/diagonal.jl
+++ b/stdlib/LinearAlgebra/test/diagonal.jl
@@ -433,4 +433,11 @@ end
     @test Diagonal(transpose([1, 2, 3])) == Diagonal([1 2 3])
 end
 
+@testset "Multiplication with Adjoint and Transpose vectors (#26863)" begin
+    x = rand(5)
+    D = Diagonal(rand(5))
+    @test x'*D*x == (x'*D)*x == (x'*Array(D))*x
+    @test Transpose(x)*D*x == (Transpose(x)*D)*x == (Transpose(x)*Array(D))*x
+end
+
 end # module TestDiagonal