diff --git a/crates/bin/pd/src/migrate.rs b/crates/bin/pd/src/migrate.rs
index c793dc2c37..3a4a8e7095 100644
--- a/crates/bin/pd/src/migrate.rs
+++ b/crates/bin/pd/src/migrate.rs
@@ -89,7 +89,13 @@ impl Migration {
             }
 
             Migration::Testnet78 => {
-                testnet78::migrate(storage, pd_home.clone(), genesis_start).await?
+                testnet78::migrate(storage, pd_home.clone(), genesis_start).await?;
+                // Testnet78 migration munges CometBFT config TOML directly.
+                if let Some(comet_home) = comet_home.clone() {
+                    testnet78::update_cometbft_mempool_settings(comet_home)?;
+                } else {
+                    tracing::warn!("cometbft home not specified, update 'max_txs_bytes' value manually before restarting");
+                }
             }
             // We keep historical migrations around for now, this will help inform an abstracted
             // design. Feel free to remove it if it's causing you trouble.
diff --git a/crates/bin/pd/src/migrate/testnet78.rs b/crates/bin/pd/src/migrate/testnet78.rs
index 642cbbb5f6..a3341b7244 100644
--- a/crates/bin/pd/src/migrate/testnet78.rs
+++ b/crates/bin/pd/src/migrate/testnet78.rs
@@ -520,8 +520,50 @@ fn truncate(s: &str, max_bytes: usize) -> &str {
     &s[..closest]
 }
 
-mod tests {
+/// Edit the node's CometBFT config file to set mempool.max_tx_bytes to the required value.
+/// This value will affect consensus, but the config setting is specified for CometBFT
+/// specifically.
+#[instrument]
+pub(crate) fn update_cometbft_mempool_settings(cometbft_home: PathBuf) -> anyhow::Result<()> {
+    let cometbft_config_path = cometbft_home.join("config").join("config.toml");
+    use std::fs::OpenOptions;
+    use std::io::Write;
+    use tendermint_config::TendermintConfig;
+    tracing::debug!(cometbft_config_path = %cometbft_config_path.display(), "opening cometbft config file");
+    let mut cometbft_config = TendermintConfig::load_toml_file(&cometbft_config_path)
+        .context("failed to load pre-migration cometbft config file")?;
+    // We expect this value to be '1073741824', assuming it's unedited from `pd testnet join`.
+    let pre_migration_value = cometbft_config.mempool.max_txs_bytes;
+    // The new value was updated in GH4594 and represents ~10MB or (2^20 * 10).
+    let post_migration_value = 10485760;
+    // Set new value
+    cometbft_config.mempool.max_txs_bytes = post_migration_value;
+    // Overwrite file
+    let mut fh = OpenOptions::new()
+        .create(false)
+        .write(true)
+        .truncate(true)
+        .open(cometbft_config_path.clone())
+        .context("failed to open cometbft config file for writing")?;
+    fh.write_all(toml::to_string(&cometbft_config)?.as_bytes())
+        .context("failed to write updated cometbft config to toml file")?;
+
+    // These asseratiosn are left over from debugging, can be removed.
+    let config_munged = TendermintConfig::load_toml_file(&cometbft_config_path)?;
+    let new_block_size = config_munged.mempool.max_txs_bytes;
+    assert_ne!(
+        new_block_size, pre_migration_value,
+        "post-migration max_txs_bytes remains pre-migration value"
+    );
+    assert_eq!(
+        new_block_size, post_migration_value,
+        "post-migration max_txs_bytes value not correctly updated"
+    );
 
+    Ok(())
+}
+
+mod tests {
     #[test]
     fn truncation() {
         use super::truncate;