Tammi Smith, 37, was being taken to Tempe police headquarters and later will be booked into Maricopa County jail on charges of custodial interference, conspiracy to commit custodial interference and forgery, Sgt. Steve Carbajal said.
Smith and her husband Jack Smith have been considered "persons of interest" for weeks in the investigation into the disappearance of 8-month-old Gabriel Johnson of Tempe.
Jack Smith did not answer his cell phone, and his voicemail box was full. It's unclear whether they have a lawyer.
Gabriel's mother, 23-year-old Elizabeth Johnson, was arrested Dec. 30 in Florida and is charged in Arizona with kidnapping, child abuse and custodial interference. Police say she drove Gabriel to San Antonio from Tempe, stayed about a week, then took a bus to Florida without him.
She has refused to say where the baby is, but told Gabriel's father she killed him and threw his body in a trash bin. She also has said she gave the baby away to a couple in San Antonio.
The Smiths had sought to adopt Gabriel from Johnson, who gave the couple temporary guardianship over the baby for about 10 days in December before she picked him up and left Arizona.
Gabriel's father, Logan McQueary, has said the Smiths continuously pushed him to surrender his parental rights to them, but he never agreed. The Smiths have denied that.
Results from a lie-detector test taken by the Smiths last month were inconclusive; police also searched the couple's home.
Tammi Smith told The Associated Press last week that she had nothing to hide.
"Since the beginning I've been asking police to look through our computer and get whatever they want because it proves my innocence, so I'm not concerned," she said.
Investigators have seemingly been at a standstill in the case, saying they're getting fewer leads to follow up on the longer Gabriel is missing. They're also less optimistic that Gabriel is alive.
Managers of a San Antonio landfill have blocked off an area that might be searched for the baby, but police said they hadn't yet searched the site and decline to say whether they plan to do so. A portion of the Tessman Road Landfill was cordoned off based on where Gabriel was last seen on Dec. 26 in San Antonio.
