diff --git a/src/arch/x86_64/CodeGen.zig b/src/arch/x86_64/CodeGen.zig
index f4dbd502e7..2c7751cff5 100644
--- a/src/arch/x86_64/CodeGen.zig
+++ b/src/arch/x86_64/CodeGen.zig
@@ -10147,9 +10147,46 @@ fn airAggregateInit(self: *Self, inst: Air.Inst.Index) !void {
 fn airUnionInit(self: *Self, inst: Air.Inst.Index) !void {
     const ty_pl = self.air.instructions.items(.data)[inst].ty_pl;
     const extra = self.air.extraData(Air.UnionInit, ty_pl.payload).data;
-    _ = extra;
-    return self.fail("TODO implement airUnionInit for x86_64", .{});
-    //return self.finishAir(inst, result, .{ extra.init, .none, .none });
+    const result: MCValue = result: {
+        const union_ty = self.air.typeOfIndex(inst);
+        const layout = union_ty.unionGetLayout(self.target.*);
+
+        const src_ty = self.air.typeOf(extra.init);
+        const src_mcv = try self.resolveInst(extra.init);
+        if (layout.tag_size == 0) {
+            if (self.reuseOperand(inst, extra.init, 0, src_mcv)) break :result src_mcv;
+
+            const dst_mcv = try self.allocRegOrMem(inst, true);
+            try self.genCopy(src_ty, dst_mcv, src_mcv);
+            break :result dst_mcv;
+        }
+
+        const dst_mcv = try self.allocRegOrMem(inst, false);
+
+        const union_obj = union_ty.cast(Type.Payload.Union).?.data;
+        const field_name = union_obj.fields.keys()[extra.field_index];
+        const tag_ty = union_ty.unionTagTypeSafety().?;
+        const field_index = @intCast(u32, tag_ty.enumFieldIndex(field_name).?);
+        var tag_pl = Value.Payload.U32{ .base = .{ .tag = .enum_field_index }, .data = field_index };
+        const tag_val = Value.initPayload(&tag_pl.base);
+        var tag_int_pl: Value.Payload.U64 = undefined;
+        const tag_int_val = tag_val.enumToInt(tag_ty, &tag_int_pl);
+        const tag_int = tag_int_val.toUnsignedInt(self.target.*);
+        const tag_off = if (layout.tag_align < layout.payload_align)
+            @intCast(i32, layout.payload_size)
+        else
+            0;
+        try self.genCopy(tag_ty, dst_mcv.address().offset(tag_off).deref(), .{ .immediate = tag_int });
+
+        const pl_off = if (layout.tag_align < layout.payload_align)
+            0
+        else
+            @intCast(i32, layout.tag_size);
+        try self.genCopy(src_ty, dst_mcv.address().offset(pl_off).deref(), src_mcv);
+
+        break :result dst_mcv;
+    };
+    return self.finishAir(inst, result, .{ extra.init, .none, .none });
 }
 
 fn airPrefetch(self: *Self, inst: Air.Inst.Index) !void {
diff --git a/test/behavior/bugs/13366.zig b/test/behavior/bugs/13366.zig
index cec015cc40..8419fbe265 100644
--- a/test/behavior/bugs/13366.zig
+++ b/test/behavior/bugs/13366.zig
@@ -14,7 +14,6 @@ const Block = struct {
 
 test {
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_x86) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
diff --git a/test/behavior/struct.zig b/test/behavior/struct.zig
index 797a22c3a8..659acbf56b 100644
--- a/test/behavior/struct.zig
+++ b/test/behavior/struct.zig
@@ -1352,7 +1352,6 @@ test "struct field init value is size of the struct" {
 }
 
 test "under-aligned struct field" {
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
diff --git a/test/behavior/tuple.zig b/test/behavior/tuple.zig
index 2ce1922e50..b6fde88af2 100644
--- a/test/behavior/tuple.zig
+++ b/test/behavior/tuple.zig
@@ -367,7 +367,6 @@ test "branching inside tuple literal" {
 test "tuple initialized with a runtime known value" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
 
diff --git a/test/behavior/union.zig b/test/behavior/union.zig
index 26a5d060df..b03e40214f 100644
--- a/test/behavior/union.zig
+++ b/test/behavior/union.zig
@@ -821,7 +821,6 @@ test "return union init with void payload" {
 }
 
 test "@unionInit stored to a const" {
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
@@ -994,7 +993,6 @@ test "function call result coerces from tagged union to the tag" {
 }
 
 test "cast from anonymous struct to union" {
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
@@ -1028,7 +1026,6 @@ test "cast from anonymous struct to union" {
 }
 
 test "cast from pointer to anonymous struct to pointer to union" {
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
@@ -1199,7 +1196,6 @@ test "global variable struct contains union initialized to non-most-aligned fiel
 test "union with no result loc initiated with a runtime value" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
 
@@ -1217,7 +1213,6 @@ test "union with no result loc initiated with a runtime value" {
 test "union with a large struct field" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
 
@@ -1288,7 +1283,6 @@ test "extern union most-aligned field is smaller" {
 }
 
 test "return an extern union from C calling convention" {
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
@@ -1397,7 +1391,6 @@ test "union and enum field order doesn't match" {
 }
 
 test "@unionInit uses tag value instead of field index" {
-    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO