Brown will work without pay to help 38-year-old manager Gary Locke, who will start his first full season in the job with the club on minus 15 points following their descent into administration.
The 62-year-old had two coaching spells at Tynecastle as right-hand man to Jim Jefferies, the first incorporating a Scottish Cup win in 1998 when Locke was a player, and the second ending when the pair were removed three games into the 2011-12 season.
Locke also worked under Jefferies and Brown at Bradford and Kilmarnock and began his coaching career at Tynecastle when the pair were in charge.
Locke told the club's official website: "It's an absolute no-brainer for me.
"It's not often you get the chance to get someone of Billy's experience and he's offered to come in and help me out during pre-season which is a magnificent gesture.
"I can't do it all on my own and it's obvious I'll need a help so to have Billy in here is a great help.
"I can't afford to bring in an assistant and it's a measure of the man that Billy was only too willing to help out without getting paid.
"He's someone I obviously know very well and I'm delighted to have him here. We are all in this together."
Brown was assistant manager at Hibernian after leaving Hearts, a stint which ended with the 5-1 Edinburgh derby William Hill Scottish Cup final defeat, before managing East Fife last season.
- Sports & Recreation
- Gary Locke
- Jim Jefferies