The £2m project is aimed at tourists and people visiting the town on business.
Wetherspoon spokesman Eddie Gershon said: “We are very keen to open a hotel at our pub, The Postal Order, and are pleased that planning permission has been granted.
“We believe the pub will benefit the pub and, importantly, benefit Blackburn itself.
It highlights Wetherspoon’s commitment to the pub, its staff and its customers and also to the town. We believe the new hotel will bring an additional 15 jobs. There are no on-site or opening dates as yet.”