diff --git a/src/postgres/math_udfs.rs b/src/postgres/math_udfs.rs
index 97bf072..b8d4d15 100644
--- a/src/postgres/math_udfs.rs
+++ b/src/postgres/math_udfs.rs
@@ -5,7 +5,7 @@ use datafusion::arrow::datatypes::DataType;
 use datafusion::common::DataFusionError;
 use datafusion::error::Result;
 
-/// Inverse cosine, result in radians.
+/// Inverse cosine, result in degrees.
 pub fn acosd(args: &[ArrayRef]) -> Result<ArrayRef> {
     let values = datafusion::common::cast::as_float64_array(&args[0])?;
     let mut float64array_builder = Float64Array::builder(args[0].len());
@@ -37,6 +37,38 @@ pub fn acosd(args: &[ArrayRef]) -> Result<ArrayRef> {
     Ok(Arc::new(float64array_builder.finish()) as ArrayRef)
 }
 
+/// Inverse cosine, result in radians.
+pub fn acos(args: &[ArrayRef]) -> Result<ArrayRef> {
+    let values = datafusion::common::cast::as_float64_array(&args[0])?;
+    let mut float64array_builder = Float64Array::builder(args[0].len());
+
+    values.iter().try_for_each(|value| {
+        if let Some(value) = value {
+            if value > 1.0 {
+                return Err(DataFusionError::Internal(
+                    "input is out of range".to_string(),
+                ));
+            }
+            let result = value.acos();
+            if result.fract() < 0.9 {
+                if result.fract() < 0.01 {
+                    float64array_builder.append_value(result.floor());
+                } else {
+                    float64array_builder.append_value(result);
+                }
+            } else {
+                float64array_builder.append_value(result.ceil());
+            }
+            Ok::<(), DataFusionError>(())
+        } else {
+            float64array_builder.append_null();
+            Ok::<(), DataFusionError>(())
+        }
+    })?;
+
+    Ok(Arc::new(float64array_builder.finish()) as ArrayRef)
+}
+
 /// Nearest integer greater than or equal to argument (same as ceil).
 pub fn ceiling(args: &[ArrayRef]) -> Result<ArrayRef> {
     let values = datafusion::common::cast::as_float64_array(&args[0])?;
@@ -238,6 +270,63 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn test_acos() -> Result<()> {
+        let ctx = register_udfs_for_test()?;
+        let df = ctx.sql("select acos(0.5) as col_result").await?;
+
+        let batches = df.clone().collect().await?;
+
+        let expected: Vec<&str> = r#"
++--------------------+
+| col_result         |
++--------------------+
+| 1.0471975511965976 |
++--------------------+"#
+            .split('\n')
+            .filter_map(|input| {
+                if input.is_empty() {
+                    None
+                } else {
+                    Some(input.trim())
+                }
+            })
+            .collect();
+        assert_batches_sorted_eq!(expected, &batches);
+
+        let df = ctx.sql("select acos(0.4) as col_result").await?;
+
+        let batches = df.clone().collect().await?;
+
+        let expected: Vec<&str> = r#"
++--------------------+
+| col_result         |
++--------------------+
+| 1.1592794807274085 |
++--------------------+"#
+            .split('\n')
+            .filter_map(|input| {
+                if input.is_empty() {
+                    None
+                } else {
+                    Some(input.trim())
+                }
+            })
+            .collect();
+        assert_batches_sorted_eq!(expected, &batches);
+
+        let df = ctx.sql("select acos(1.4) as col_result").await?;
+
+        let result = df.clone().collect().await;
+        assert!(result
+            .err()
+            .unwrap()
+            .to_string()
+            .contains("input is out of range"));
+
+        Ok(())
+    }
+
     #[tokio::test]
     async fn test_ceiling() -> Result<()> {
         let ctx = register_udfs_for_test()?;
diff --git a/src/postgres/mod.rs b/src/postgres/mod.rs
index d085dbb..4c2113b 100644
--- a/src/postgres/mod.rs
+++ b/src/postgres/mod.rs
@@ -2,7 +2,7 @@
 
 use std::sync::Arc;
 
-use crate::postgres::math_udfs::{acosd, ceiling, div, erf, erfc};
+use crate::postgres::math_udfs::{acos, acosd, ceiling, div, erf, erfc};
 use crate::postgres::network_udfs::{
     broadcast, family, host, hostmask, inet_merge, inet_same_family, masklen, netmask, network,
     set_masklen,
@@ -24,6 +24,7 @@ pub fn register_postgres_udfs(ctx: &SessionContext) -> Result<()> {
 
 fn register_math_udfs(ctx: &SessionContext) -> Result<()> {
     register_acosd(ctx);
+    register_acos(ctx);
     register_ceiling(ctx);
     register_erf(ctx);
     register_erfc(ctx);
@@ -44,6 +45,19 @@ fn register_acosd(ctx: &SessionContext) {
     ctx.register_udf(acosd_udf);
 }
 
+fn register_acos(ctx: &SessionContext) {
+    let acos_udf = make_scalar_function(acos);
+    let return_type: ReturnTypeFunction = Arc::new(move |_| Ok(Arc::new(Float64)));
+    let acos_udf = ScalarUDF::new(
+        "acos",
+        &Signature::uniform(1, vec![Float64], Volatility::Immutable),
+        &return_type,
+        &acos_udf,
+    );
+
+    ctx.register_udf(acos_udf);
+}
+
 fn register_ceiling(ctx: &SessionContext) {
     let ceiling_udf = make_scalar_function(ceiling);
     let return_type: ReturnTypeFunction = Arc::new(move |_| Ok(Arc::new(Float64)));
diff --git a/supports/postgres.md b/supports/postgres.md
index d6aff06..750e597 100644
--- a/supports/postgres.md
+++ b/supports/postgres.md
@@ -37,7 +37,7 @@ https://www.postgresql.org/docs/16/functions-math.html
 | ✅           | acosd ( double precision ) → double precision                                              | Inverse cosine, result in degrees                                                                                                                                                             | acosd(0.5) → 60                         | 
 | ❓           | asind ( double precision ) → double precision                                              | Inverse sine, result in degrees                                                                                                                                                               | asind(0.5) → 30                         | 
 | ❓           | atand ( double precision ) → double precision                                              | Inverse tangent, result in degrees                                                                                                                                                            | atand(1) → 45                           | 
-| ❓           | cosd ( double precision ) → double precision                                               | Cosine, argument in degrees                                                                                                                                                                   | cosd(60) → 0.5                          | 
+| ✅           | cosd ( double precision ) → double precision                                               | Cosine, argument in degrees                                                                                                                                                                   | cosd(60) → 0.5                          | 
 | ❓           | cotd ( double precision ) → double precision                                               | Cotangent, argument in degrees                                                                                                                                                                | cotd(45) → 1                            | 
 | ❓           | sind ( double precision ) → double precision                                               | Sine, argument in degrees                                                                                                                                                                     | sind(30) → 0.5                          | 
 | ❓           | tand ( double precision ) → double precision                                               | Tangent, argument in degrees                                                                                                                                                                  | tand(45) → 1                            |