diff --git a/crates/prek/src/cli/mod.rs b/crates/prek/src/cli/mod.rs
index 371b16e37..6bbc221af 100644
--- a/crates/prek/src/cli/mod.rs
+++ b/crates/prek/src/cli/mod.rs
@@ -734,9 +734,10 @@ pub(crate) enum UtilCommand {
 
 #[derive(Debug, Args)]
 pub(crate) struct YamlToTomlArgs {
-    /// The YAML configuration file to convert.
+    /// The YAML configuration file to convert. If omitted, discovers
+    /// `.pre-commit-config.yaml` or `.pre-commit-config.yml` in the current directory.
     #[arg(value_name = "CONFIG", value_hint = ValueHint::FilePath)]
-    pub(crate) input: PathBuf,
+    pub(crate) input: Option<PathBuf>,
 
     /// Path to write the generated prek.toml file.
     /// Defaults to `prek.toml` in the same directory as the input file.
diff --git a/crates/prek/src/cli/yaml_to_toml.rs b/crates/prek/src/cli/yaml_to_toml.rs
index 6e9658d44..e0b8f15b9 100644
--- a/crates/prek/src/cli/yaml_to_toml.rs
+++ b/crates/prek/src/cli/yaml_to_toml.rs
@@ -4,7 +4,7 @@ use std::path::{Path, PathBuf};
 
 use anyhow::{Context, Result};
 use owo_colors::OwoColorize;
-use prek_consts::PREK_TOML;
+use prek_consts::{PRE_COMMIT_CONFIG_YAML, PRE_COMMIT_CONFIG_YML, PREK_TOML};
 use toml_edit::{Array, ArrayOfTables, DocumentMut, InlineTable, Table, Value};
 
 use crate::cli::ExitStatus;
@@ -12,16 +12,45 @@ use crate::config;
 use crate::fs::Simplified;
 use crate::printer::Printer;
 
+/// Resolve the input config path, falling back to `.pre-commit-config.yaml` or
+/// `.pre-commit-config.yml` in the current directory.
+fn resolve_input(input: Option<PathBuf>) -> Result<PathBuf> {
+    if let Some(path) = input {
+        return Ok(path);
+    }
+
+    let yaml = Path::new(PRE_COMMIT_CONFIG_YAML);
+    if yaml.is_file() {
+        return Ok(yaml.to_path_buf());
+    }
+
+    let yml = Path::new(PRE_COMMIT_CONFIG_YML);
+    if yml.is_file() {
+        return Ok(yml.to_path_buf());
+    }
+
+    anyhow::bail!(
+        "No `{}` or `{}` found in the current directory\n\n\
+         {} Provide a path explicitly: {}",
+        PRE_COMMIT_CONFIG_YAML.cyan(),
+        PRE_COMMIT_CONFIG_YML.cyan(),
+        "hint:".yellow().bold(),
+        "prek util yaml-to-toml <CONFIG>".cyan()
+    );
+}
+
 pub(crate) fn yaml_to_toml(
-    input: &Path,
+    input: Option<PathBuf>,
     output: Option<PathBuf>,
     force: bool,
     printer: Printer,
 ) -> Result<ExitStatus> {
+    let input = resolve_input(input)?;
+
     // Validate the input file first.
-    let _ = config::load_config(input)?;
+    let _ = config::load_config(&input)?;
 
-    let content = fs_err::read_to_string(input)?;
+    let content = fs_err::read_to_string(&input)?;
     let value: serde_json::Value = serde_saphyr::from_str(&content)?;
 
     let output = output.unwrap_or_else(|| input.parent().unwrap_or(Path::new(".")).join(PREK_TOML));
@@ -65,7 +94,8 @@ pub(crate) fn yaml_to_toml(
 
     writeln!(
         printer.stdout(),
-        "Written to `{}`",
+        "Converted `{}` → `{}`",
+        input.simplified_display().cyan(),
         output.simplified_display().cyan()
     )?;
 
diff --git a/crates/prek/src/main.rs b/crates/prek/src/main.rs
index 611536b31..21a0d7b02 100644
--- a/crates/prek/src/main.rs
+++ b/crates/prek/src/main.rs
@@ -401,7 +401,7 @@ async fn run(cli: Cli) -> Result<ExitStatus> {
             UtilCommand::YamlToToml(args) => {
                 show_settings!(args);
 
-                cli::yaml_to_toml(&args.input, args.output, args.force, printer)
+                cli::yaml_to_toml(args.input, args.output, args.force, printer)
             }
             UtilCommand::GenerateShellCompletion(args) => {
                 show_settings!(args);
diff --git a/crates/prek/tests/yaml_to_toml.rs b/crates/prek/tests/yaml_to_toml.rs
index f6a1f1218..ea8983e53 100644
--- a/crates/prek/tests/yaml_to_toml.rs
+++ b/crates/prek/tests/yaml_to_toml.rs
@@ -1,6 +1,6 @@
 use assert_fs::assert::PathAssert;
 use assert_fs::fixture::{FileWriteStr, PathChild};
-use prek_consts::PREK_TOML;
+use prek_consts::{PRE_COMMIT_CONFIG_YAML, PRE_COMMIT_CONFIG_YML, PREK_TOML};
 
 use crate::common::{TestContext, cmd_snapshot};
 
@@ -68,14 +68,14 @@ fn yaml_to_toml_writes_default_output() -> anyhow::Result<()> {
         context
             .command()
             .args(["util", "yaml-to-toml", "config.yaml"]),
-        @r#"
+        @"
     success: true
     exit_code: 0
     ----- stdout -----
-    Written to `prek.toml`
+    Converted `config.yaml` → `prek.toml`
 
     ----- stderr -----
-    "#
+    "
     );
 
     insta::assert_snapshot!(context.read(PREK_TOML), @r#"
@@ -180,7 +180,7 @@ fn yaml_to_toml_force_overwrite() -> anyhow::Result<()> {
     success: true
     exit_code: 0
     ----- stdout -----
-    Written to `prek.toml`
+    Converted `config.yaml` → `prek.toml`
 
     ----- stderr -----
     "
@@ -252,3 +252,133 @@ fn yaml_to_toml_same_output() -> anyhow::Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn yaml_to_toml_discovers_pre_commit_config_yaml() -> anyhow::Result<()> {
+    let context = TestContext::new();
+
+    context
+        .work_dir()
+        .child(PRE_COMMIT_CONFIG_YAML)
+        .write_str(YAML_CONFIG)?;
+
+    cmd_snapshot!(
+        context.filters(),
+        context.command().args(["util", "yaml-to-toml"]),
+        @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Converted `.pre-commit-config.yaml` → `prek.toml`
+
+    ----- stderr -----
+    "
+    );
+
+    context
+        .work_dir()
+        .child(PREK_TOML)
+        .assert(predicates::path::exists());
+
+    Ok(())
+}
+
+#[test]
+fn yaml_to_toml_discovers_pre_commit_config_yml() -> anyhow::Result<()> {
+    let context = TestContext::new();
+
+    context
+        .work_dir()
+        .child(PRE_COMMIT_CONFIG_YML)
+        .write_str(YAML_CONFIG)?;
+
+    cmd_snapshot!(
+        context.filters(),
+        context.command().args(["util", "yaml-to-toml"]),
+        @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Converted `.pre-commit-config.yml` → `prek.toml`
+
+    ----- stderr -----
+    "
+    );
+
+    context
+        .work_dir()
+        .child(PREK_TOML)
+        .assert(predicates::path::exists());
+
+    Ok(())
+}
+
+#[test]
+fn yaml_to_toml_prefers_yaml_over_yml() -> anyhow::Result<()> {
+    let context = TestContext::new();
+
+    // Write different content to each file so we can verify which was used.
+    let yaml_only = indoc::indoc! {r"
+        repos:
+          - repo: builtin
+            hooks:
+              - id: trailing-whitespace
+    "};
+    let yml_only = indoc::indoc! {r"
+        repos:
+          - repo: builtin
+            hooks:
+              - id: end-of-file-fixer
+    "};
+
+    context
+        .work_dir()
+        .child(PRE_COMMIT_CONFIG_YAML)
+        .write_str(yaml_only)?;
+    context
+        .work_dir()
+        .child(PRE_COMMIT_CONFIG_YML)
+        .write_str(yml_only)?;
+
+    cmd_snapshot!(
+        context.filters(),
+        context.command().args(["util", "yaml-to-toml"]),
+        @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Converted `.pre-commit-config.yaml` → `prek.toml`
+
+    ----- stderr -----
+    "
+    );
+
+    // The .yaml file contains trailing-whitespace, the .yml contains end-of-file-fixer.
+    let output = context.read(PREK_TOML);
+    assert!(
+        output.contains("trailing-whitespace"),
+        "Expected .yaml to be preferred over .yml"
+    );
+
+    Ok(())
+}
+
+#[test]
+fn yaml_to_toml_error_when_no_config_found() {
+    let context = TestContext::new();
+
+    cmd_snapshot!(
+        context.filters(),
+        context.command().args(["util", "yaml-to-toml"]),
+        @r#"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: No `.pre-commit-config.yaml` or `.pre-commit-config.yml` found in the current directory
+
+    hint: Provide a path explicitly: prek util yaml-to-toml <CONFIG>
+    "#
+    );
+}
diff --git a/docs/cli.md b/docs/cli.md
index 0f649809a..63e2415b5 100644
--- a/docs/cli.md
+++ b/docs/cli.md
@@ -914,12 +914,12 @@ Convert a YAML configuration file to prek.toml
 <h3 class="cli-reference">Usage</h3>
 
 ```
-prek util yaml-to-toml [OPTIONS] <CONFIG>
+prek util yaml-to-toml [OPTIONS] [CONFIG]
 ```
 
 <h3 class="cli-reference">Arguments</h3>
 
-<dl class="cli-reference"><dt id="prek-util-yaml-to-toml--input"><a href="#prek-util-yaml-to-toml--input"><code>CONFIG</code></a></dt><dd><p>The YAML configuration file to convert</p>
+<dl class="cli-reference"><dt id="prek-util-yaml-to-toml--input"><a href="#prek-util-yaml-to-toml--input"><code>CONFIG</code></a></dt><dd><p>The YAML configuration file to convert. If omitted, discovers <code>.pre-commit-config.yaml</code> or <code>.pre-commit-config.yml</code> in the current directory</p>
 </dd></dl>
 
 <h3 class="cli-reference">Options</h3>