Francis, 28, goes straight into Nottinghamshire's squad for the Friends Provident match against Derbyshire on Monday after impressing during a pre-season trial period with the club.
"We only had six seamers on the staff at the start of the season and with Ryan Sidebottom playing for England and AJ Harris injured," said director of cricket Mick Newell.
"We couldn't operate with just four quick bowlers. Simon is an experienced bowler who has bowled well for us in the second XI this year and will give us cover in an area where we are thin. He can slot straight into the team if necessary and there's every chance he will play on Monday."
Francis started his career with Hampshire before moving to Somerset in 2002. He toured Malaysia and India with England A in 2003-04 but limited Championship opportunities in 2006 led to his release.
"After being released by Somerset I had to make plans for life away from cricket and it made me appreciate how lucky professional cricketers are," said Francis, who has since worked in sales and marketing. "I didn't want to get to the age of 33 and wish I had given county cricket one more go and my love for cricket has been reinvigorated in the last four months.
"I'm determined to make the most of this opportunity but I've got no expectations beyond mucking in with the squad and doing my very best for Nottinghamshire over the rest of the season."