The 26-year-old leg-spinner has found his opportunities limited at his boyhood county, despite starting the season with a five-wicket haul in the first round of this season's County Championship against Surrey. He then played one match on loan at Durham before returning for Lancashire's Vitality Blast campaign.
Nevertheless, Parkinson still boasts exceptional first-class and Twenty20 records, with 151 dismissals at an average of 25.92 and 139 at 18.96 respectively.
"The time is right for me to embrace a new challenge and environment," said Parkinson in a Kent statement. "I'm so excited that this will be at Kent.
"I'm proud of my record in the game so far & I believe my best years as a leg-spin bowler are ahead of me. I've heard very positive things about the squad and I'm looking forward to working with Matt Walker and the other coaches.
"I promise my new teammates and the Kent supporters that I will throw myself into this new challenge and I can't wait to get down to Canterbury to get started.
"I'd also like to take this opportunity to thank the Lancs members for the support they have given to me during my time at Old Trafford. I thank my teammates for their friendship and wish them the best."
Kent's Director of Cricket, Paul Downton, said: "Everyone at Kent is really excited that Matt has decided to join the club on a long-term contract.
"To have a proven match-winning leg-spinner in our ranks will be a major boost to the squad. Matt is also a feisty competitor who will bring a lot to our dressing room. We look forward to welcoming him to Canterbury."
Mark Chilton, Lancashire's Director of Cricket Performance, said: "On behalf of everybody connected with Lancashire Cricket, I would like to thank Matt for all of his commitment and efforts since making his first-class debut in 2016, having progressed through our academy.
"Matt has found first-team opportunities limited this season and we respect the decision that he has made to move on at the end of his contract, in order to seek a new challenge elsewhere. We would all like to wish Matt well for the next stage of his career at Canterbury."
It seems unlikely Parkinson will move on loan for the remainder of the season, as is usually the way with these sorts of deals. The fact Lancashire (seventh) and Kent (ninth) are separated by 20 points at the foot of Division One makes that an awkward proposition.