diff --git a/NEWS.md b/NEWS.md
index cdf940755907f..a188d85bf7e7e 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -367,6 +367,9 @@ Deprecated or removed
     Instead, reshape the array or add trailing indices so the dimensionality and number of indices
     match ([#14770], [#23628]).
 
+  * `fill!(A::Diagonal, x)` and `fill!(A::AbstractTriangular, x)` have been deprecated
+    in favor of `Base.LinAlg.fillslots!(A, x)` ([#24413]).
+
   * Using Bool values directly as indices is now deprecated and will be an error in the future. Convert
     them to `Int` before indexing if you intend to access index `1` for `true` and `0` for `false`.
 
diff --git a/base/deprecated.jl b/base/deprecated.jl
index 9927285ba7d26..4d613246b7cf6 100644
--- a/base/deprecated.jl
+++ b/base/deprecated.jl
@@ -1886,6 +1886,10 @@ end
 # After deprecation is removed, enable the @testset "indexing by Bool values" in test/arrayops.jl
 # Also un-comment the new definition in base/indices.jl
 
+# deprecate odd fill! methods
+@deprecate fill!(D::Diagonal, x)                       fillslots!(D, x)
+@deprecate fill!(A::Base.LinAlg.AbstractTriangular, x) fillslots!(A, x)
+
 function diagm(v::BitVector)
     depwarn(string("diagm(v::BitVector) is deprecated, use diagm(0 => v) or ",
         "BitMatrix(Diagonal(v)) instead"), :diagm)
diff --git a/base/linalg/bidiag.jl b/base/linalg/bidiag.jl
index 4a1f05ab222ef..68d1aa076ae09 100644
--- a/base/linalg/bidiag.jl
+++ b/base/linalg/bidiag.jl
@@ -607,10 +607,6 @@ function fillslots!(A::SpecialArrays, x)
     return A
 end
 
-# for historical reasons:
-fill!(a::AbstractTriangular, x) = fillslots!(a, x)
-fill!(D::Diagonal, x) = fillslots!(D, x)
-
 _small_enough(A::Bidiagonal) = size(A, 1) <= 1
 _small_enough(A::Tridiagonal) = size(A, 1) <= 2
 _small_enough(A::SymTridiagonal) = size(A, 1) <= 2
diff --git a/base/linalg/diagonal.jl b/base/linalg/diagonal.jl
index 93533a4b02f64..1582371f17a91 100644
--- a/base/linalg/diagonal.jl
+++ b/base/linalg/diagonal.jl
@@ -61,6 +61,11 @@ convert(::Type{Array}, D::Diagonal) = convert(Matrix, D)
 similar(D::Diagonal, ::Type{T}) where {T} = Diagonal(similar(D.diag, T))
 similar(D::Diagonal, ::Type{T}, dims::Union{Dims{1},Dims{2}}) where {T} = spzeros(T, dims...)
 
+Base.zeros(D::Diagonal) = Diagonal(fill!(similar(D.diag), 0))
+Base.zeros(D::Diagonal, ::Type{T}) where {T} = Diagonal(fill!(similar(D, T), 0))
+Base.zeros(D::Diagonal, ::Type{T}, dims::Dims) where {T} = fill!(similar(D, T, dims), 0)
+Base.zeros(D::Diagonal, ::Type{T}, dims::Integer...) where {T} = fill!(similar(D, T, dims), 0)
+
 copy!(D1::Diagonal, D2::Diagonal) = (copy!(D1.diag, D2.diag); D1)
 
 size(D::Diagonal) = (length(D.diag),length(D.diag))
diff --git a/test/linalg/triangular.jl b/test/linalg/triangular.jl
index 862fa75f184e3..7c1d7c547e528 100644
--- a/test/linalg/triangular.jl
+++ b/test/linalg/triangular.jl
@@ -34,9 +34,6 @@ for elty1 in (Float32, Float64, BigFloat, Complex64, Complex128, Complex{BigFloa
         # full!
         @test full!(copy(A1)) == A1
 
-        # fill!
-        @test full!(fill!(copy(A1), 1)) == t1(ones(size(A1)...))
-
         # similar
         @test isa(similar(A1), t1)
         @test eltype(similar(A1)) == elty1