Humphreys’ primary job is as an assistant coach to Wales head coach Wayne Pivac and the pair are currently preparing for a busy autumn campaign which includes back-to-back fixtures against New Zealand, South Africa, Fiji and Australia.
After that series is complete, Rennie and Humphreys will reunite to work with the Barbarians men’s team for their clash with Samoa at Twickenham on November 27.
The pair previously worked together at Scottish side Glasgow Warriors between 2017 and 2020.
Humphreys said: “The Barbarians is steeped with so much history, especially in Wales with the 1973 game and the Easter tours.
“It is special to represent the club as a player, but to now coach them is an even greater responsibility.
“I look forward to joining up with Dave and hopefully get the team to put in a special performance on the day and finish off the autumn internationals in style.”
The Barbarians men’s side face Samoa at 2.30pm on November 27 as part of a double header of fixtures.
The Barbarian Women will take on a Springboks Women’s XV following the men’s match at 5.15pm.
Rennie, currently coaching Australia in the Rugby Championship ahead of his team arriving in Cardiff for a game with Pivac’s Wales on November 20, is looking forward to working with Humphreys.
The pair will go from working on opposite coaching sides when Wales meet the Wallabies to being compatriots in successive weekends.
“Humphs is an excellent coach and I look forward to working with him again,” said Rennie.
“It’s a huge honour to be invited to coach the Barbarians later this year and it should be a great occasion at Twickenham.
“The Barbarians are renowned for bringing together leading players from around the world and the challenge for coaches is moulding them quickly into a team that can play expansive entertaining rugby for the fans to enjoy.”
Three experienced, world-class players have also been confirmed to put on the Barbarians shirt against Samoa.
They are 74 cap back-row and former Argentina captain Pablo Matera, who led the Pumas to a landmark first victory against the All Blacks in the 2020 Rugby Championship and giant South African forwards Duane Vermeulen and Malcom Marx.
All three have represented the Barbarians before in matches at Twickenham.
“We are in the process of assembling a great squad and I’m delighted Duane, Pablo and Malcolm have all accepted to play,” said Rennie.
“Further player announcements will be made in the coming weeks.”
