Garcia Pimenta’s side were without a number of key players for the crucial match but were still able to pick up an important win.
Ronald Araujo, Riqui Puig and Ansu Fati all played for the first team at Alaves, while Alex Collado has been ruled out for three months with a fractured metatarsal.
Yet Barca B got off to the perfect start when Konrad de la Fuente put them ahead in the second minute after a great run from Dani Morer.
Their lead did not last too long as Miguel de la Fuente equalised for Valladolid Promesas seven minutes later.
However, Barca B restored their lead just before the hour mark. Again Morer was involved with a cross for captain Monchu to volley home.
Goalkeeper Inaki Pena was called on to make a great save from Carlos Doncel with 10 minutes to go before Konrad struck again late on with his second of the match.
This time it was Ilaix Moriba who was the creator out on the left, slipping the ball to Konrad inside the penalty area to smash home.
Yet there was still time for more drama. Miguel Rubio pulled one back with a header in the 89th minute to set up a nervy finish but Barca managed to see it out to keep their promotion hopes alive.