The Ecuador winger moved to Teesside on loan in January as a precursor to a more permanent deal.
Boro have now announced that will happen as planned when the summer transfer window opens.
Sarmiento headed to the Riverside hoping to complete a promotion hat-trick having helped Ipswich and Burnley to the Premier League in the previous seasons.
However, he only got on as sub near the end of the ‘Spygate’ final as Boro went down 1-0 to Hull City.
Middlesbrough only scored one goal in three games during a play-off campaign made infamous when Southampton were spotted spying on them during one of their training sessions.
Sarmiento, who turns 24 next month, spent the first half of the season with Serie A strugglers Cremonese, who were eventually relegated.
He was seen as part of Middlesbrough’s push for automatic promotion when he returned to England.
But he only made five starts and came off the bench on 14 occasions, two of which were in the play-offs.
He missed out on the Ecuador squad for the World Cup finals.
The former Benfica and Charlton prospect will now look to make a more sustained contribution next season.
