Sarmiento will become Boro’s third signing of the January window, with his arrival likely to be confirmed by the club on Monday.
It will be an initial loan with an obligation for the deal to be made permanent for £3.2m.
Sarmiento will further strengthen an attack that has already been boosted by the arrival of Leo Castledine this month. The arrival of the winger will delight boss Kim Hellberg, who says he’s admired the 23-year-old since first watching him play for Brighton in the Premier League.
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Sarmiento has extensive Championship experience having spent time on loan with West Brom, Ipswich and Burnley in the last two seasons, winning promotion with the latter two clubs.
Boro identified Sarmiento as a leading target this month but early last week looked ready to walk away having failed to reach an agreement with Brighton.
Birmingham City then struck a deal with Albion but Sarmiento is understood to have made clear his preference was a move to the Riverside, prompting the Seagulls to bring Boro back to the negotiating table.
A £3.2m deal was then agreed between the clubs. Sarmiento will join on an initial loan with an obligation for Boro to make the move permanent for the agreed fee. It’s understood Sarmiento had his medical on Sunday.
The winger has spent the first half of the season on loan with Cremonese but hasn’t featured for the Italian club since early November.
Sarmiento will have a full week on the training pitch with his new teammates, with Boro not in action again until next Saturday when they host Norwich City.