Real Betis took the friendly Weston Cup as Middlesbrough tasted defeat for the first time since March in the first match of this year's Marbella trip.
Jordan Rhodes scored Boro's only goal with a tidy header in the second half, after Dani Ceballos and Ruben Castro had given the La Liga side a commanding lead.
Antonio Barragan made his Boro debut from the first whistle but there was still no place in the squad for Victor Valdes, who will be hoping to feature in one of the further two friendlies to come as Boro continue their preparations ahead of the Premier League season.
Superior Betis take deserved first-half lead
Betis should have been ahead inside the first minute after Marten de Roon was caught napping inside the defensive third, but Dimi Konstantopoulos got down quickly to make an instinctive save.
Middlesbrough didn't make a chance in the opening 25 minutes and saw little of the ball, and the scoreline was made to reflect Betis' superiority as Dani Ceballos produced a neat close-range finish past the goalkeeper after an incisive pass from the right.
Five minutes later, the visitors created their first opening of the game and almost capitalised on it, with David Nugent hitting the post and seeing his follow-up effort deflect wide off the goalkeeper.
Things threatened to boil over in the intense heat on the stroke of half-time as a de Roon foul prompted a furious reaction, and both sets of players gathered on the half-way line with some minor pushing and shoving, but no major confrontation.
Altered line-up improves in second half
Boro made ten changes at half time with Adam Reach the only player surviving for the second half, and their play improved almost immediately.
Early in the half, Viktor Fischer had the ball in the net but was aware of the offside flag before he knocked the ball home after being sent clear on goal.
With 65 minutes played, Betis wanted a penalty as George Friend, one of the half-time replacements, tangled with an attacker at the edge of the six-yard box, but only a goal kick was given.
It took only five minutes after that incident for Betis to double their lead, as Ruben Castro reacted well to fire in a smart finish after a low cross from the right wasn't properly defended.
Boro responded immediately with a cross of their own, as Emilio Nsue delivered an excellent ball from deep towards the back post. Jordan Rhodes was on hand to steer a cushioned header back across goal and inside the side-netting to halve the deficit and give his side hope of a comeback.
However, while Middlesbrough - playing in their new red home kits for the first time - pressed more than in the first half, there were no clear-cut opportunities and the game fizzled out.
Positives despite negative result
Aitor Karanka will be disappointed with a negative result but, as he said recently, the friendly games at this stage are more about fitness than tactics or performance.
Nevertheless, the goalscoring form of Rhodes and the quality exhibited by Adam Clayton in the first half and George Friend in the second will be a source of encouragement, with none of the trio looking out of place against top-flight opposition.
Middlesbrough fans will also be pleased with the return of Gaston Ramirez, who came on for the final ten minutes after confirming his comeback with a work permit complete.