diff --git a/CHANGELOG.md b/CHANGELOG.md
index dbe4d77a1..ce1879f1c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,9 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+### Fixed
+- Pip requirements.txt parser failing with third-party registries
+
 ## [5.7.2] - 2023-10-10
 
 ### Fixed
diff --git a/lockfile/src/parsers/pypi.rs b/lockfile/src/parsers/pypi.rs
index 86ed643f8..4064e3b5a 100644
--- a/lockfile/src/parsers/pypi.rs
+++ b/lockfile/src/parsers/pypi.rs
@@ -11,14 +11,15 @@ use nom::Err as NomErr;
 use phylum_types::types::package::PackageType;
 
 use crate::parsers::{self, IResult};
-use crate::{Package, PackageVersion};
+use crate::{Package, PackageVersion, ThirdPartyVersion};
 
 pub fn parse(mut input: &str) -> IResult<&str, Vec<Package>> {
     let mut pkgs = Vec::new();
 
+    let mut registry = None;
     while !input.is_empty() {
         // Get the next line.
-        let (new_input, line) = line(input)?;
+        let (new_input, line) = line(input, &mut registry)?;
         input = new_input;
 
         // Ignore empty lines.
@@ -30,7 +31,7 @@ pub fn parse(mut input: &str) -> IResult<&str, Vec<Package>> {
         let (_, line) = alt((take_until(" #"), rest))(line)?;
 
         // Parse dependency.
-        let (_, pkg) = package(line)?;
+        let (_, pkg) = package(line, registry)?;
         pkgs.push(pkg);
     }
 
@@ -38,7 +39,7 @@ pub fn parse(mut input: &str) -> IResult<&str, Vec<Package>> {
 }
 
 /// Parse one line in the lockfile.
-fn line(input: &str) -> IResult<&str, &str> {
+fn line<'a>(input: &'a str, registry: &mut Option<&'a str>) -> IResult<&'a str, &'a str> {
     // Take everything until the next newline.
     //
     // This takes line continuation characters into account.
@@ -52,10 +53,26 @@ fn line(input: &str) -> IResult<&str, &str> {
         line = "";
     }
 
+    // Ignore index config options.
+    //
+    // Since `ThirdPartyVersion` only allows a single registry, we only record the
+    // primary one.
+    if let Some(index_url) = line
+        .strip_prefix("--index-url")
+        .and_then(|line| line.strip_prefix(['=', ' ']))
+        .or_else(|| line.strip_prefix("-i"))
+    {
+        *registry = Some(index_url.trim());
+        line = "";
+    }
+    if line.starts_with("--extra-index-url") {
+        line = "";
+    }
+
     Ok((input, line))
 }
 
-fn package(input: &str) -> IResult<&str, Package> {
+fn package<'a>(input: &'a str, registry: Option<&str>) -> IResult<&'a str, Package> {
     // Ignore everything after `;`.
     let (_, input) = alt((take_until(";"), rest))(input)?;
 
@@ -85,7 +102,13 @@ fn package(input: &str) -> IResult<&str, Package> {
 
     // Parse first-party dependencies.
     let (input, version) = package_version(input)?;
-    let version = PackageVersion::FirstParty(version.trim().into());
+    let version = match registry {
+        Some(registry) => PackageVersion::ThirdParty(ThirdPartyVersion {
+            version: version.trim().into(),
+            registry: registry.into(),
+        }),
+        None => PackageVersion::FirstParty(version.trim().into()),
+    };
 
     // Ensure line is empty after the dependency.
     line_done(input)?;
diff --git a/lockfile/src/python.rs b/lockfile/src/python.rs
index 7cee3eda4..09b84b4f0 100644
--- a/lockfile/src/python.rs
+++ b/lockfile/src/python.rs
@@ -238,7 +238,7 @@ mod tests {
         let pkgs = PyRequirements
             .parse(include_str!("../../tests/fixtures/requirements-locked.txt"))
             .unwrap();
-        assert_eq!(pkgs.len(), 12);
+        assert_eq!(pkgs.len(), 14);
 
         let expected_pkgs = [
             Package {
@@ -301,6 +301,22 @@ mod tests {
                 version: PackageVersion::Path(Some("/tmp/editable".into())),
                 package_type: PackageType::PyPi,
             },
+            Package {
+                name: "other-registry-a".into(),
+                version: PackageVersion::ThirdParty(ThirdPartyVersion {
+                    registry: "https://mirror1.phylum.io/simple/".into(),
+                    version: "3.2.1".into(),
+                }),
+                package_type: PackageType::PyPi,
+            },
+            Package {
+                name: "other-registry".into(),
+                version: PackageVersion::ThirdParty(ThirdPartyVersion {
+                    registry: "https://mirror2.phylum.io/simple/".into(),
+                    version: "1.2.3".into(),
+                }),
+                package_type: PackageType::PyPi,
+            },
         ];
 
         for expected_pkg in expected_pkgs {
diff --git a/tests/fixtures/requirements-locked.txt b/tests/fixtures/requirements-locked.txt
index 29142950b..07297db1c 100644
--- a/tests/fixtures/requirements-locked.txt
+++ b/tests/fixtures/requirements-locked.txt
@@ -32,3 +32,10 @@ tomli @ https://files.pythonhosted.org/packages/97/75/10a9ebee3fd790d20926a90a25
 -e git+ssh://git@github.com/phylum-dev/phylum-ci.git@7d6d859ad368d1ab0a933f24679e3d3c08a40eac#egg=phylum
 
 -e /tmp/editable ; python_version >= "3.7" and python_version < "3.12"
+
+--index-url https://unused.phylum.io/simple/
+--index-url=https://mirror1.phylum.io/simple/
+other-registry-a==3.2.1
+-ihttps://mirror2.phylum.io/simple/
+--extra-index-url=https://mirror3.phylum.io/simple/
+other-registry==1.2.3