Smith, who turns 24 on Wednesday, was questioned by police in San Jose after an automobile accident involving a single vehicle was reported about 7 a.m. A police report says officers questioned Smith and gave him a breath test.
"They conducted an investigation and determined he was under the influence,” Sgt. Heather Randol told the Associated Press.
The 49ers released a statement saying the "organization is aware of the matter involving Aldon Smith. As we continue to gather information regarding this situation, we will have no further comment."
[Updated, 1:45 p.m.: Santa Clara County Sheriff's Sgt. Kurtis Stenderup told the Associted Press that Smith was released from jail Friday morning after he posted $5,250 bail.]
Smith, who had 19½ sacks to earn his first
Smith also was reportedly arrested on suspicion of DUI in Miami Beach in January 2012.
Smith, who is 6-foot-4 and 260 pounds, was a first-round draft pick from Missouri who made a splash as a rookie with 14 sacks in 2011.
Before his second season, though, Ronndale Esporlas claims in a suit filed in Santa Clara County Superior Court that he was shot twice in the leg while attending a party at Smith's home in San Jose. Esporlas said that Smith and Walker charged for admission and drinks.
Esporlas said in the suit that Smith and Walker fired gunshots from a balcony in an attempt to end the party. Esporlas said in the suit that he was shot in a leg later in the driveway of the home when he was caught between crossfire of two groups firing weapons.
Smith and Walker, who could still face criminal charges in the incident, have denied causing injuries to Esporlas.
Wire service and Internet reports contributed to this story.