The 27-year-old forward is back in the National League after ending his stay at Ross County.
Effiong went north of the border when he left Woking in January.
He scored eight goals in 28 National League outings for the Cards last season before his move to Scotland.
He is Dover's first summer signing after a challenging close season.
A number of players have moved on, with last season’s top scorer Ryan Bird signing for Maidenhead United after rejecting a new deal.
Midfielder Mitch Pinnock has signed for AFC Wimbledon, and defender Manny Parry has switch to Boreham Wood.
But one player who is staying in winger Kadell Daniel, who has agreed a new deal at Crabble.