Bracknell town mayor Cllr Alvin Finch will officially open the bar, in Stanley Walk, at 6pm, followed by music from a live band.
This temporary establishment will be run by Michael Theal - former manager of the now-closed Bull pub - but will be superseded by an RBL branch-run club in the autumn.
Branch chaplin Sue Marshall-Halls said: "It's really good to have this because people keep asking us when we're going to be open again. Now we can say 'here it is!' "
The Bracknell RBL club was forced to close back in March because it was making a loss.
Ms Marshall-Halls stressed the Legion club is still defunct but that the new bar is being done privately, with support from the branch. RBL members and serving military personnel will have free entry. Temporary members can pay a fee of 50p per visit.
Opening in the first week will be: today (Friday), 6pm-late; Saturday noon-late;
Sunday, noon-10.30pm; Monday noon-late. Then it will be open from August 1, Thursdays-Tuesdays, 10am-late.