The 28-year-old centre came through the ranks with the Warriors before moving to London Scottish in 2016. He returned to Scotstoun for a short spell last term but left at the end of a Covid-blighted campaign in which he did not get the opportunity to play in front of the Glasgow crowd.
He has spent the intervening period representing Great Britain 7s at the Olympics in the summer as well as playing for Scotland 7s.
Fergusson will return to Glasgow on a full-time basis in three months.
He told the Warriors website: "My family are here, and Glasgow has been my home club and the club I've supported since I was a boy so it was a really easy decision for me when the offer of a contract came through.
"I'm looking forward to continuing to play for GB7s up until the New Year and then focusing all of my attention on Glasgow and hopefully trying to get myself back in a Warriors jersey.
"I just want to play as much rugby as possible and this contract is going to allow me to do that.
"I'm buzzing to be back - I've not had the chance to play in front of the Scotstoun crowd in nearly seven years, and I'm looking forward to pushing for my place in the 23 and to run out in front of my friends and family at Scotstoun."
Fergusson, who overcame Hodgkin's Lymphoma at the age of 20, made his most recent appearance for Glasgow in a 46-25 home victory over Benetton in March.
Warriors head coach Danny Wilson said: "Robbie played really well for us last season and took his opportunities when they came.
"We were all pleased to see him earn his spot in the GB7s squad for the Olympics and more recently the 2021 World 7s Series.
"He is a quality player, a strong leader, a great communicator, and a popular member of our squad. We're looking forward to having him back and competing for the jersey."