diff --git a/src/psbt.h b/src/psbt.h
--- a/src/psbt.h
+++ b/src/psbt.h
@@ -36,6 +36,10 @@
 // The separator has no value.
 static constexpr uint8_t PSBT_SEPARATOR = 0x00;
 
+// BIP 174 does not specify a maximum file size, but we set a limit anyway
+// to prevent reading a stream indefinitely and running out of memory.
+const std::streamsize MAX_FILE_SIZE_PSBT = 100000000; // 100 MiB
+
 /** A structure for PSBTs which contain per-input information */
 struct PSBTInput {
     CTxOut utxo;