The grenade was thrown at the porch of the bar in Mexico's second most populous city, police said on Saturday.
Three women and three men were killed in the attack and more than 20 people were wounded. The assailants fled in several cars.
This is the second grenade attack on a Guadalajara bar in less than a month: On January 16 an argument between gunmen and musicians ended in a blast that killed two people.
The latest attack took place just hours after a shootout between soldiers and presumed cartel gunmen left eight people, including an innocent driver, dead in the northeastern city of Monterrey.
Monterrey is at the intersection of several highways - often used as drug smuggling routes - heading north into the US.
Two rival drug organisations, the Gulf cartel and their former allies, the Zetas, are battling for control of the area.
The incidents in Monterrey and Guadalajara brought the overall death toll of this weekend's drug-related violence to at least 30.
In Chihuahua state, 11 people were killed in several separate murders in Ciudad Juarez, Mexico's deadliest city, across from the US city of El Paso, Texas between late Friday and Saturday, authorities said.
And gunmen shot dead five men together in an additional group killing on a highway between Chihuahua city, the state capital, and Ciudad Juarez, police said.
Grenade Mexican Nightclub
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