The ex-Gloucester head coach will take on the role as interim forwards coach, assisting interim head coach Scott Johnson.
SRU chief executive Mark Dodson said: "Dean will be with us for 12 weeks and will bring all his top-flight coaching savvy and experience to benefit our forward pack."
Johnson was appointed caretaker boss last month after Andy Robinson's resignation followed the November loss to Tonga which left Scotland 12th in the International Board Rugby world rankings.
The 46-year-old Ryan and former Scotland hooker Stevie Scott, who has been appointed interim skills coach, complete the coaching team for the Six Nations, which for Scotland begins on February 2 at Twickenham against England.
The duo will start work on Monday as part of Johnson's coaching group, which also includes defence coach Matt Taylor and kicking coach Duncan Hodge and scrum coach Massimo Cuttitta.
Johnson told scottishrugby.org: "I'm delighted we have secured the services of Dean and Stevie for this year's championship.
"To get people of their calibre mid-season is a real coup. Dean is a sound, knowledgeable, mature rugby man and Stevie, with his background of having played 11 times for Scotland, knows what it means to wear the jersey.
"With Matt, Hodgey and Massimo already fully engaged in Six Nations planning, we've now a management team in place for the start of the championship next month."