diff --git a/ops/opset13/acos.go b/ops/opset13/acos.go
new file mode 100644
index 0000000..9e766c0
--- /dev/null
+++ b/ops/opset13/acos.go
@@ -0,0 +1,75 @@
+package opset13
+
+import (
+	"math"
+
+	"github.com/advancedclimatesystems/gonnx/onnx"
+	"github.com/advancedclimatesystems/gonnx/ops"
+	"gorgonia.org/tensor"
+)
+
+// Acos represents the ONNX acos operator.
+type Acos struct{}
+
+// newAcos creates a new acos operator.
+func newAcos() ops.Operator {
+	return &Acos{}
+}
+
+// Init initializes the acos operator.
+func (c *Acos) Init(_ []*onnx.AttributeProto) error {
+	return nil
+}
+
+// Apply applies the acos operator.
+func (c *Acos) Apply(inputs []tensor.Tensor) ([]tensor.Tensor, error) {
+	var (
+		out tensor.Tensor
+		err error
+	)
+
+	switch inputs[0].Dtype() {
+	case tensor.Float32:
+		out, err = inputs[0].Apply(acos[float32])
+	case tensor.Float64:
+		out, err = inputs[0].Apply(acos[float64])
+	default:
+		return nil, ops.ErrInvalidInputType(0, inputs[0].Dtype().String(), c)
+	}
+
+	if err != nil {
+		return nil, err
+	}
+
+	return []tensor.Tensor{out}, nil
+}
+
+// ValidateInputs validates the inputs that will be given to Apply for this operator.
+func (c *Acos) ValidateInputs(inputs []tensor.Tensor) ([]tensor.Tensor, error) {
+	return ops.ValidateInputs(c, inputs)
+}
+
+// GetMinInputs returns the minimum number of input tensors this operator expects.
+func (c *Acos) GetMinInputs() int {
+	return 1
+}
+
+// GetMaxInputs returns the maximum number of input tensors this operator expects.
+func (c *Acos) GetMaxInputs() int {
+	return 1
+}
+
+// GetInputTypeConstraints returns a list. Every element represents a set of allowed tensor dtypes
+// for the corresponding input tensor.
+func (c *Acos) GetInputTypeConstraints() [][]tensor.Dtype {
+	return [][]tensor.Dtype{{tensor.Float32, tensor.Float64}}
+}
+
+// String implements the stringer interface, and can be used to format errors or messages.
+func (c *Acos) String() string {
+	return "acos operator"
+}
+
+func acos[T ops.FloatType](x T) T {
+	return T(math.Acos(float64(x)))
+}
diff --git a/ops/opset13/acos_test.go b/ops/opset13/acos_test.go
new file mode 100644
index 0000000..e2c755a
--- /dev/null
+++ b/ops/opset13/acos_test.go
@@ -0,0 +1,99 @@
+package opset13
+
+import (
+	"testing"
+
+	"github.com/advancedclimatesystems/gonnx/ops"
+	"github.com/stretchr/testify/assert"
+	"gorgonia.org/tensor"
+)
+
+func TestAcosInit(t *testing.T) {
+	c := &Acos{}
+
+	// since 'acos' does not have any attributes we pass in nil. This should not
+	// fail initializing the acos.
+	err := c.Init(nil)
+	assert.Nil(t, err)
+}
+
+func TestAcos(t *testing.T) {
+	tests := []struct {
+		acos     *Acos
+		backing  []float32
+		shape    []int
+		expected []float32
+	}{
+		{
+			&Acos{},
+			[]float32{-1, -1, 0, 1},
+			[]int{2, 2},
+			[]float32{3.1415927, 3.1415927, 1.5707964, 0},
+		},
+		{
+			&Acos{},
+			[]float32{1, 0.5, 0.0, -0.5},
+			[]int{1, 4},
+			[]float32{0, 1.0471976, 1.5707964, 2.0943952},
+		},
+		{
+			&Acos{},
+			[]float32{-1, -1, -1, -1},
+			[]int{1, 4},
+			[]float32{3.1415927, 3.1415927, 3.1415927, 3.1415927},
+		},
+	}
+
+	for _, test := range tests {
+		inputs := []tensor.Tensor{
+			ops.TensorWithBackingFixture(test.backing, test.shape...),
+		}
+
+		res, err := test.acos.Apply(inputs)
+		assert.Nil(t, err)
+
+		assert.Nil(t, err)
+		assert.Equal(t, test.expected, res[0].Data())
+	}
+}
+
+func TestInputValidationAcos(t *testing.T) {
+	tests := []struct {
+		inputs []tensor.Tensor
+		err    error
+	}{
+		{
+			[]tensor.Tensor{
+				ops.TensorWithBackingFixture([]float32{1, 2}, 2),
+			},
+			nil,
+		},
+		{
+			[]tensor.Tensor{
+				ops.TensorWithBackingFixture([]float64{1, 2}, 2),
+			},
+			nil,
+		},
+		{
+			[]tensor.Tensor{},
+			ops.ErrInvalidInputCount(0, &Acos{}),
+		},
+		{
+			[]tensor.Tensor{
+				ops.TensorWithBackingFixture([]int{1, 2}, 2),
+			},
+			ops.ErrInvalidInputType(0, "int", &Acos{}),
+		},
+	}
+
+	for _, test := range tests {
+		acos := &Acos{}
+		validated, err := acos.ValidateInputs(test.inputs)
+
+		assert.Equal(t, test.err, err)
+
+		if test.err == nil {
+			assert.Equal(t, test.inputs, validated)
+		}
+	}
+}
diff --git a/ops/opset13/opset13.go b/ops/opset13/opset13.go
index 953ca27..7384a05 100644
--- a/ops/opset13/opset13.go
+++ b/ops/opset13/opset13.go
@@ -6,6 +6,7 @@ import (
 
 var operators13 = map[string]func() ops.Operator{
 	"Abs":             newAbs,
+	"Acos":            newAcos,
 	"Acosh":           newAcosh,
 	"Add":             newAdd,
 	"Cast":            newCast,
diff --git a/ops/opset13/opset13_test.go b/ops/opset13/opset13_test.go
index cb27818..5a2fc35 100644
--- a/ops/opset13/opset13_test.go
+++ b/ops/opset13/opset13_test.go
@@ -18,6 +18,11 @@ func TestGetOperator(t *testing.T) {
 			newAbs(),
 			nil,
 		},
+		{
+			"Acos",
+			newAcos(),
+			nil,
+		},
 		{
 			"Acosh",
 			newAcosh(),
diff --git a/ops_test.go b/ops_test.go
index f54a2e6..df7ea8e 100644
--- a/ops_test.go
+++ b/ops_test.go
@@ -272,6 +272,8 @@ func readTestTensors(basePath, baseFile string, inputs []*onnx.ValueInfoProto) (
 // With this we check if we truly run all tests we expected from the integration test.
 var expectedTests = []string{
 	"test_abs",
+	"test_acos",
+	"test_acos_example",
 	"test_acosh",
 	"test_acosh_example",
 	"test_add",