Sykes arrives at Chadfields from league rivals Maldon & Tiptree.
Sykes joined the Jammers in the summer from Canvey Island, where he was in his second spell.
He scored 25 times in 45 games for the Gulls last season having joined them from Braintree Town in July 2017.
The 6’4″, 24-year old joined the Barnet youth team as an under-16 and was top scorer in the English under-18 youth system in the 2011/12 season with 24 goals.
He made his senior debut at the age of 17 against Bristol Rovers in August 2012 and signed his first professional contract with the Bees in November 2012.
He won the League Two apprentice of the year award at the Football League Awards in March 2013.
During his three years with Barnet, Sykes had various loan spells before joining Dartford for the 2015/16, making 25 appearances.
After a second short loan spell at Canvey Island, Sykes made the move permanent, joining the then-Isthmian League Premier Division side for the 2016/17 season where he scored 21 goals in a difficult season for the club as they were relegated.
The Dockers have also signed Darren Phillips from league rivals Basildon United.
The defender has previously played for East Thurrock United, Burnham Ramblers and Basildon Town and has made 13 appearances for United this season.