Marvin Solano's men have been involved in a battle with Deportivo Saprissa at the top and both have held first place at stages this season.
But after Saprissa drew 1-1 at Limon on Saturday, Herediano took advantage with a 4-2 victory at home to Santos de Guapiles.
Herediano (30 points) sit just ahead of Saprissa (29), while Alajuelense (22) have a game in hand in third.
In Heredia on Sunday, mid-table Santos de Guapiles stunned the hosts thanks to Antonio Salazar opening the scoring on five minutes.
But Solano's side were 3-1 ahead before half-time as Esteban Granados, Victor Nunez and Francisco Calvo netted.
Nunez scored his ninth league goal of the season to make it 4-1 on 79 minutes, before Juan Solis netted a consolation effort for the visitors.
Saprissa were forced to come from behind to take a point from fifth-placed Limon.
Mayron George's 12th-minute opener was cancelled out by Carlos Saucedo in the first half as the teams shared the spoils.
Alajuelense were also held to a 1-1 draw as Puntarenas came from behind to clinch a point.
Fourth-placed UCR drew level on points with Alajuelense thanks to a 2-1 win at Belen Siglo XXI, while Uruguay were 2-0 winners at home to Cartagines.
Perez Zeledon's struggles continued as Francisco Rodriguez's first-half brace saw Carmelita to a 3-1 home win over the league's bottom side.
