The back-rower, who has made 19 appearances for the Warriors, will link up with the Greene King IPA Championship side ahead of the new campaign, with the terms of his loan deal allowing him to be recalled to Scotstoun at any time.
Glasgow coach Kenny Myrray said: "It's been a tough year for Lewis (Wynne), but people should remember that it wasn't long ago that he played 10 games on the bounce for us at the start of the season (in 2016/17), so he's proved that he can compete at this level.
"Getting some regular game time down at London Scottish, in a league that we know to be very tough, will benefit him, and I'm sure he'll do well down there."
Speaking to the London Scottish website, Wynne said: "I'm buzzing to be joining the squad this pre season and putting the work in before a hard year ahead. I'm always looking to further my game and this is an amazing opportunity with a great team. As a player, I will give my all to contribute positively to what will be a successful season for Scottish."