Jenkins was sacked after defeat to Fiji saw Wales eliminated in the group stages of the World Cup and he has kept a low profile since.
But following the departure of Alex Codling from the London Welsh coaching staff, Jenkins has accepted an invitation to put the players through their paces at Old Deer Park this week.
Club chief executive Peter Thomas said: "We're delighted to have Gareth helping us prepare for a very important game this weekend.
"He's an outstanding coach with a great deal of experience, and I'm sure his involvement will give the guys a tremendous lift as we go through our preparations.
"We are delighted that Gareth has found time in his busy schedule to assist us, and having lost Alex it's great to be able to call on someone of Gareth's experience."