Barca and Betis have both confirmed the deal is done, with only a medical and formal signing of the contract left to complete before the 25-year-old is presented at his new club.
The Camp Nou club confirmed they have sold Tello for an initial fee of €4m, with a further €1m payable in add-ons, while they will financially benefit if the winger later moves on elsewhere.
Tello joined Barca from city rivals Espanyol in 2010, making his debut for the first-team in November 2011.
He established himself as a role player for Barca, playing 44 La Liga games in two seasons between 2012 and 2014, although the second of those campaigns saw him used increasingly as a substitute.
Tello has spent the last three seasons out on loan, spending 18 months each with Porto and Fiorentina respectively, the Betis move coming to fruition after the Viola turned down their option to buy him.
Tello's departure comes on the same day Barca re-signed Gerard Deulofeu from Everton for a reported €12m fee.