diff --git a/src/oneelement.jl b/src/oneelement.jl
index 556f692c..ed375f98 100644
--- a/src/oneelement.jl
+++ b/src/oneelement.jl
@@ -392,6 +392,14 @@ function triu(A::OneElementMatrix, k::Integer=0)
     OneElement(nzband < k ? zero(A.val) : A.val, A.ind, axes(A))
 end
 
+# diag
+function diag(O::OneElementMatrix, k::Integer=0)
+    Base.require_one_based_indexing(O)
+    len = length(diagind(O, k))
+    ind = O.ind[2] - O.ind[1] == k ? (k >= 0 ? O.ind[2] - k : O.ind[1] + k) : len + 1
+    OneElement(getindex_value(O), ind, len)
+end
+
 # broadcast
 
 function broadcasted(::DefaultArrayStyle{N}, ::typeof(conj), r::OneElement{<:Any,N}) where {N}
diff --git a/test/runtests.jl b/test/runtests.jl
index 0163df8a..ad5f50a7 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -2699,6 +2699,16 @@ end
         B = OneElement(2, (1, 2), (Base.IdentityUnitRange(1:1), Base.IdentityUnitRange(2:2)))
         @test repr(B) == "OneElement(2, (1, 2), (Base.IdentityUnitRange(1:1), Base.IdentityUnitRange(2:2)))"
     end
+
+    @testset "diag" begin
+        @testset for sz in [(0,0), (0,1), (1,0), (1,1), (4,4), (4,6), (6,3)], ind in CartesianIndices(sz)
+            O = OneElement(4, Tuple(ind), sz)
+            @testset for k in -maximum(sz):maximum(sz)
+                @test diag(O, k) == diag(Array(O), k)
+                @test diag(O, k) isa OneElement{Int,1}
+            end
+        end
+    end
 end
 
 @testset "repeat" begin