Oscar Ramirez's side put the pressure on Herediano with a 3-1 win over Santos de Guapiles drawing them level on points.
Herediano were unable to collect three points against Cartagines on Sunday, held to a 0-0 draw away from home.
Alajuelense took just four minutes to break the deadlock in front of their home fans at the Estadio Alejandro Morera Soto.
Jose Ortiz powered a right-footed finish into the bottom corner in a one-on-on with goalkeeper Adrian De Lemos after being put through by a headed pass.
The lead was doubled eight minutes before half-time thanks to Johan Venegas, who was left unmarked to head in a corner from six yards.
The hosts struck again nine minutes into the second half as Osvaldo Rodriguez slid in to poke in a cross at the back post.
Things went from bad to worse for the visitors on 64 minutes when Javier Loaiza was shown a straight red card.
Despite being down to 10 men, Santos de Guapiles pulled a goal back through Kevin Fajardo, who volleyed in at the back post with 11 minutes remaining.
While Alajuelense are a point behind Herediano, Santos de Guapiles sit eighth after six matches this league campaign.
Deportivo Saprissa extended their unbeaten run to five matches with a 2-1 victory at home to Puntarenas, who had goal-scorer Luciano Bostal sent off late on.
Belen Siglo XXI were also reduced to 10 men in their 2-0 win at Carmelita, Limon edged UCR 1-0 and Perez Zeledon and Uruguay played out a 1-1 draw.
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