diff --git a/Cargo.lock b/Cargo.lock
index 211fa3d51..9800d77be 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2688,6 +2688,7 @@ dependencies = [
  "apple-sdk",
  "aws-config",
  "aws-sdk-s3",
+ "base64 0.22.1",
  "bytes",
  "clap",
  "duct",
diff --git a/Cargo.toml b/Cargo.toml
index 13557b8b0..be8b93342 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,6 +9,7 @@ anyhow = "1.0.100"
 apple-sdk = "0.6.0"
 aws-config = { version = "1", features = ["behavior-version-latest"] }
 aws-sdk-s3 = "1"
+base64 = "0.22"
 bytes = "1.11.0"
 clap = "4.5.52"
 duct = "1.1.1"
diff --git a/src/macho.rs b/src/macho.rs
index 5b3155164..2173ee7b6 100644
--- a/src/macho.rs
+++ b/src/macho.rs
@@ -4,7 +4,7 @@
 
 use {
     crate::validation::ValidationContext,
-    anyhow::{anyhow, Context, Result},
+    anyhow::{Context, Result, anyhow},
     apple_sdk::{AppleSdk, SdkSearch, SdkSearchLocation, SdkSorting, SdkVersion, SimpleSdk},
     semver::Version,
     std::{
diff --git a/src/s3.rs b/src/s3.rs
index 92ebac4b7..09d356782 100644
--- a/src/s3.rs
+++ b/src/s3.rs
@@ -4,12 +4,14 @@
 
 use {
     crate::release::build_wanted_filenames,
-    anyhow::{Result, anyhow},
+    anyhow::{Result, anyhow, ensure},
     aws_sdk_s3::primitives::ByteStream,
+    base64::{Engine as _, engine::general_purpose::STANDARD as BASE64},
     clap::ArgMatches,
     futures::{StreamExt, TryStreamExt},
+    sha2::{Digest, Sha256},
     std::{
-        collections::BTreeSet,
+        collections::{BTreeMap, BTreeSet},
         path::{Path, PathBuf},
     },
 };
@@ -21,12 +23,88 @@ const UPLOAD_CONCURRENCY: usize = 4;
 /// The AWS SDK uses exponential backoff with jitter between attempts.
 const S3_MAX_ATTEMPTS: u32 = 5;
 
+/// A validated SHA-256 checksum.
+#[derive(Clone, Debug, Eq, PartialEq)]
+struct Sha256Sum([u8; 32]);
+
+impl Sha256Sum {
+    fn from_bytes(bytes: [u8; 32]) -> Self {
+        Self(bytes)
+    }
+
+    fn to_base64(&self) -> String {
+        BASE64.encode(self.0)
+    }
+}
+
+impl TryFrom<&str> for Sha256Sum {
+    type Error = anyhow::Error;
+
+    fn try_from(hex_digest: &str) -> Result<Self> {
+        let bytes = hex::decode(hex_digest)?;
+        let bytes: [u8; 32] = bytes
+            .try_into()
+            .map_err(|_| anyhow!("expected 32-byte sha256 digest"))?;
+        Ok(Self(bytes))
+    }
+}
+
+/// Parse a `SHA256SUMS` file into a map of filename → digest.
+fn parse_sha256sums(content: &str) -> Result<BTreeMap<String, Sha256Sum>> {
+    let mut digests = BTreeMap::new();
+
+    for (line_no, line) in content.lines().enumerate() {
+        let (digest, filename) = line
+            .split_once("  ")
+            .ok_or_else(|| anyhow!("malformed SHA256SUMS line {}", line_no + 1))?;
+        ensure!(
+            !filename.is_empty(),
+            "missing filename on SHA256SUMS line {}",
+            line_no + 1
+        );
+        let digest = Sha256Sum::try_from(digest)?;
+        ensure!(
+            digests.insert(filename.to_string(), digest).is_none(),
+            "duplicate filename in SHA256SUMS: {filename}"
+        );
+    }
+
+    Ok(digests)
+}
+
+fn ensure_sha256sums_coverage(
+    wanted_filenames: &BTreeMap<String, String>,
+    present_filenames: &BTreeSet<String>,
+    sha256_digests: &BTreeMap<String, Sha256Sum>,
+) -> Result<()> {
+    let missing = wanted_filenames
+        .iter()
+        .filter(|(source, _)| present_filenames.contains(*source))
+        .map(|(_, dest)| dest)
+        .filter(|dest| !sha256_digests.contains_key(*dest))
+        .collect::<Vec<_>>();
+
+    if missing.is_empty() {
+        Ok(())
+    } else {
+        Err(anyhow!(
+            "SHA256SUMS missing {} entries; first missing artifact: {}",
+            missing.len(),
+            missing[0]
+        ))
+    }
+}
+
 /// Upload a single file to S3 under `key`, setting an immutable cache-control header.
+///
+/// When `sha256` is provided the SHA-256 content checksum is included in the
+/// PUT request so that S3 verifies data integrity on receipt.
 async fn upload_s3_file(
     s3: &aws_sdk_s3::Client,
     bucket: &str,
     key: &str,
     path: &Path,
+    sha256: Option<&Sha256Sum>,
     dry_run: bool,
 ) -> Result<()> {
     println!(
@@ -43,13 +121,18 @@ async fn upload_s3_file(
     // concurrently, so splitting each file into multipart chunks would add complexity
     // without meaningfully improving throughput.
     let body = ByteStream::from_path(path).await?;
-    s3.put_object()
+    let mut req = s3
+        .put_object()
         .bucket(bucket)
         .key(key)
         .body(body)
-        .cache_control("public, max-age=31536000, immutable")
-        .send()
-        .await?;
+        .cache_control("public, max-age=31536000, immutable");
+
+    if let Some(digest) = sha256 {
+        req = req.checksum_sha256(digest.to_base64());
+    }
+
+    req.send().await?;
     Ok(())
 }
 
@@ -117,6 +200,26 @@ pub async fn command_upload_mirror_distributions(args: &ArgMatches) -> Result<()
         .build();
     let s3 = aws_sdk_s3::Client::from_conf(s3_config);
 
+    let shasums_path = dist_dir.join("SHA256SUMS");
+
+    // Parse SHA256SUMS (written and verified by upload-release-distributions) so
+    // we can supply a content checksum on every PUT. S3 will reject the upload if
+    // the data it receives does not match, guarding against silent corruption.
+    // In dry-run mode we skip reading the file entirely so the command can still be
+    // used to validate naming and missing-artifact handling on a fresh dist/.
+    let shasums_content = if dry_run {
+        None
+    } else {
+        Some(std::fs::read_to_string(&shasums_path)?)
+    };
+    let sha256_digests = if let Some(content) = &shasums_content {
+        let sha256_digests = parse_sha256sums(content)?;
+        ensure_sha256sums_coverage(&wanted_filenames, &filenames, &sha256_digests)?;
+        sha256_digests
+    } else {
+        BTreeMap::new()
+    };
+
     // Upload all files concurrently (up to UPLOAD_CONCURRENCY in-flight at a time).
     let upload_futs = wanted_filenames
         .iter()
@@ -126,7 +229,8 @@ pub async fn command_upload_mirror_distributions(args: &ArgMatches) -> Result<()
             let bucket = bucket.clone();
             let key = format!("{prefix}{dest}");
             let path = dist_dir.join(source);
-            async move { upload_s3_file(&s3, &bucket, &key, &path, dry_run).await }
+            let sha256 = sha256_digests.get(dest).cloned();
+            async move { upload_s3_file(&s3, &bucket, &key, &path, sha256.as_ref(), dry_run).await }
         });
 
     futures::stream::iter(upload_futs)
@@ -134,10 +238,51 @@ pub async fn command_upload_mirror_distributions(args: &ArgMatches) -> Result<()
         .try_collect::<Vec<_>>()
         .await?;
 
-    // Upload the SHA256SUMS file already written (and verified) by upload-release-distributions.
-    let shasums_path = dist_dir.join("SHA256SUMS");
+    // Upload the SHA256SUMS file itself, computing its digest on the fly.
+    let shasums_sha256 = shasums_content.as_ref().map(|content| {
+        let mut hasher = Sha256::new();
+        hasher.update(content.as_bytes());
+        Sha256Sum::from_bytes(hasher.finalize().into())
+    });
     let shasums_key = format!("{prefix}SHA256SUMS");
-    upload_s3_file(&s3, bucket, &shasums_key, &shasums_path, dry_run).await?;
+    upload_s3_file(
+        &s3,
+        bucket,
+        &shasums_key,
+        &shasums_path,
+        shasums_sha256.as_ref(),
+        dry_run,
+    )
+    .await?;
 
     Ok(())
 }
+
+#[cfg(test)]
+mod tests {
+    use super::{Sha256Sum, ensure_sha256sums_coverage, parse_sha256sums};
+    use std::collections::{BTreeMap, BTreeSet};
+
+    #[test]
+    fn sha256_sum_rejects_non_sha256_lengths() {
+        assert!(Sha256Sum::try_from("abcd").is_err());
+    }
+
+    #[test]
+    fn parse_sha256sums_rejects_malformed_lines() {
+        assert!(parse_sha256sums("not-a-valid-line\n").is_err());
+    }
+
+    #[test]
+    fn ensure_sha256sums_coverage_requires_every_uploaded_artifact() {
+        let wanted_filenames =
+            BTreeMap::from([("source.tar.zst".to_string(), "dest.tar.zst".to_string())]);
+        let present_filenames = BTreeSet::from(["source.tar.zst".to_string()]);
+        let sha256_digests = BTreeMap::new();
+
+        assert!(
+            ensure_sha256sums_coverage(&wanted_filenames, &present_filenames, &sha256_digests)
+                .is_err()
+        );
+    }
+}
diff --git a/src/validation.rs b/src/validation.rs
index 2528cc25e..1edd58332 100644
--- a/src/validation.rs
+++ b/src/validation.rs
@@ -4,21 +4,21 @@
 
 use {
     crate::{json::*, macho::*},
-    anyhow::{anyhow, Context, Result},
+    anyhow::{Context, Result, anyhow},
     clap::ArgMatches,
     normalize_path::NormalizePath,
     object::{
+        Architecture, Endianness, FileKind, Object, SectionIndex, SymbolScope,
         elf::{
-            FileHeader32, FileHeader64, ET_DYN, ET_EXEC, SHN_UNDEF, STB_GLOBAL, STB_WEAK,
+            ET_DYN, ET_EXEC, FileHeader32, FileHeader64, SHN_UNDEF, STB_GLOBAL, STB_WEAK,
             STV_DEFAULT, STV_HIDDEN,
         },
-        macho::{MachHeader32, MachHeader64, LC_CODE_SIGNATURE, MH_OBJECT, MH_TWOLEVEL},
+        macho::{LC_CODE_SIGNATURE, MH_OBJECT, MH_TWOLEVEL, MachHeader32, MachHeader64},
         read::{
             elf::{Dyn, FileHeader, SectionHeader, Sym},
             macho::{LoadCommandVariant, MachHeader, Nlist, Section, Segment},
             pe::{ImageNtHeaders, PeFile, PeFile32, PeFile64},
         },
-        Architecture, Endianness, FileKind, Object, SectionIndex, SymbolScope,
     },
     once_cell::sync::Lazy,
     std::{