There's a shocking surprise witness who's about to testify in Jesse James' custody hearing -- the former V.P. of West Coast Choppers he allegedly hooked up with while married to Sandra Bullock.
TMZ has learned Merilee Gerth -- who settled a sexual harassment case against Jesse and WCC for $725,000 back in 2007 -- has arrived at the courthouse in Orange County and is set to take the stand.
We're told Jesse's ex-wife Janine Lindemulder -- who's trying to stop Jesse from taking their daughter Sunny to Texas -- believes Merilee knows intimate details about Jesse, Sunny, Sandra and WCC that are very important to Janine's case.
Stay tuned ... it's about to get ugly.
UPDATE: Score one for Jesse ... the judge shot down Janine's attempt to get Merilee on the stand!