PaySim BD is an open-source developer tool that simulates the SSLCommerz, bKash and Nagad payment gateways, so teams can build and test checkout integrations without waiting on sandbox accounts or spending real money. It mirrors each gateway's real API paths and payloads (SSLCommerz init, hosted page, IPN and validator; bKash tokenized checkout with grant, create, execute and query; Nagad RSA-signed initialize, complete and verify), so an app only swaps its base URL. Every payment's outcome is scriptable with a SIM:<scenario> marker (immediate success, fail, cancel, late or duplicate IPN, amount mismatch, pending-then-settled, timeout), and a fake clock fast-forwards delayed callbacks and expiries. A modern dashboard (dark and light themes, fully responsive) streams every request and callback live, shows a per-transaction timeline with copy-as-curl and IPN replay, and lets you override scenarios mid-flight. The hosted pages are clearly labelled as a simulator and never touch real credentials. It runs on Vercel with Neon Postgres, Neon Realtime and Neon Object Storage, self-hosts with Docker Compose, and ships a parity suite that checks responses against the documented gateway shapes. MIT licensed.