An eighth victory in eight games for the champions restored their five-point advantage over arch-rivals Real Madrid who needed a Cristiano Ronaldo goal in the fourth minute of stoppage time to secure a 3-2 triumph at Levante earlier.
Atletico Madrid, the only other team with seven wins from their opening seven matches, can join Barca on 24 points at the top of the table with a win at home to Celta Vigo on Sunday.
Valladolid forward Javi Guerra silenced the Nou Camp when he headed in a corner in the 10th minute but Barca were level four minutes later, Alexis lashing the ball home from just outside the penalty area.
Cristian Tello squared for Xavi to sidefoot the ball in seven minutes after the break before Neymar sent Alexis clear on the right in the 64th minute and he sent a precise shot into the far corner.
The Chilean returned the compliment by setting up Brazil forward Neymar to make it 4-1 six minutes later as Barca, who have also won both Champions League outings this season, continued their impressive form under new coach Gerardo Martino.
The Argentine is the first coach to win his opening eight matches in La Liga while Barca are the first team to start a campaign with eight victories since Real in the late 1980s.
"You play 10 good games, one average one and one bad one and people, the media, criticise you," Alexis told Gol TV.
"But this victory will help me and the team," added the striker who has not always convinced since joining Barca from Udinese in 2011.
"Barca are always obliged to win and by a wide margin."