diff --git a/base/linalg/diagonal.jl b/base/linalg/diagonal.jl
index 16cf79b6e4e74e..c65496959c5d1d 100644
--- a/base/linalg/diagonal.jl
+++ b/base/linalg/diagonal.jl
@@ -55,8 +55,29 @@ isposdef(D::Diagonal) = all(D.diag .> 0)
 
 factorize(D::Diagonal) = D
 
-tril(D::Diagonal,i::Integer=0) = i == 0 ? D : zeros(D)
-triu(D::Diagonal,i::Integer=0) = i == 0 ? D : zeros(D)
+istriu(D::Diagonal) = true
+istril(D::Diagonal) = true
+function triu!(D::Diagonal,k::Integer=0)
+    n = size(D,1)
+    if abs(k) > n
+        throw(ArgumentError("requested diagonal, $k, out of bounds in matrix of size ($n,$n)"))
+    elseif k == 0
+        return D
+    else
+        return zeros(D)
+    end
+end
+
+function tril!(D::Diagonal,k::Integer=0)
+    n = size(D,1)
+    if abs(k) > n
+        throw(ArgumentError("requested diagonal, $k, out of bounds in matrix of size ($n,$n)"))
+    elseif k == 0
+        return D
+    else
+        return zeros(D)
+    end
+end
 
 ==(Da::Diagonal, Db::Diagonal) = Da.diag == Db.diag
 -(A::Diagonal)=Diagonal(-A.diag)
diff --git a/test/linalg/diagonal.jl b/test/linalg/diagonal.jl
index 0d84b0e2cca3e1..6518cd1e202be6 100644
--- a/test/linalg/diagonal.jl
+++ b/test/linalg/diagonal.jl
@@ -84,10 +84,12 @@ for relty in (Float32, Float64, BigFloat), elty in (relty, Complex{relty})
     @test_approx_eq D/D2 Diagonal(D.diag./D2.diag)
 
     # test triu/tril
-    @test triu(D,1) == zeros(D)
-    @test triu(D,0) == D
-    @test tril(D,1) == zeros(D)
-    @test tril(D,0) == D
+    @test istriu(D)
+    @test istril(D)
+    @test triu!(D,1) == zeros(D)
+    @test triu!(D,0) == D
+    @test tril!(D,1) == zeros(D)
+    @test tril!(D,0) == D
 
     # factorize
     @test factorize(D) == D