Gagan, 24, has played eight NRL games since making his debut for the Cronulla-Sutherland Sharks in 2014, then going on to join the Newcastle Knights for the 2017 season.
Gagan represented the Australian Schoolboys and the NSW under 18s, and is equally adept at playing wing or centre.
Martin, 20, led the Manly-Warringah Sea Eagles under 20s side as captain throughout their premiership-winning 2017 season, showing great leadership and a tremendous work ethic in leading the side to the final Holden Cup title.
Playing lock or hooker, the Canberra junior has ties to Rabbitohs Assistant Coach David Furner from his time coaching at the Raiders, with Martin choosing to play Rugby League despite attending rugby union school St Edmund’s College, just as Furner himself had done previously.
Rabbitohs Head Coach Anthony Seibold is pleased to have both Gagan and Martin joining the squad in Season 2018.
“Jacob was the Rookie of the Year for the Sharks in 2014 and unfortunately has had an injury-interrupted few years,” Coach Seibold said.
“He really showed his potential at the end of the 2014 season and we’re giving him an opportunity to find that form again within our squad.
“Jacob will add depth to our outside backs and he will create more competition for spots in our top 17 which is very important for any NRL squad.
“We’re all looking forward to seeing what Jacob can achieve here at Souths.”
“Jesse comes to us following a very successful year leading Manly’s under 20s team to a Grand Final win, and we’ll be helping him take the next step up to the NRL level over the 2018 pre-season.
“He’s renowned for being a very hard worker, especially in all of the effort areas of the game, and we’re looking forward to seeing Jesse progress and develop with this opportunity.
“He’ll be given every chance throughout our pre-season training to step up and we’re all looking forward to working with him.”