The 25-year-old joins the Darts having spent last season in the National League at Gateshead and Torquay United.
The former Hull City and Bristol City youngster has plenty of experience in non-League football, having played for Hayes & Yeading United and Welling United.
After loans at AFC Wimbledon, Cheltenham Town and a season with Torquay, he signed for Dover Athletic in September 2015 following a trial with Scottish Premier League side Inverness Caledonian Thistle.
However, he returned to Torquay in January 2016 after spending the first half of the season with Dover, making forty-nine starts in his two spells with the Gulls.
He moved north to join Gateshead in June 2016 after turning down a new deal with Torquay.
Ajala goes into the squad for the home game tomorrow against Oxford City.