Supt. Dan Kinsella said Dellen Millard was arrested while driving on Cawthra Road in Mississauga and charged with forcible confinement and theft over $5,000.
Kinsella said the man was a suspect previously identified by police as having the word "AMBITION" tattooed on his wrist and was arrested "without incident."
He said the second suspect in the case had not been located and Bosma's whereabouts were still unknown. Police would not say when the arrest took place or whether they have located Bosma's truck.
Kinsella said police had received over 300 tips from the public and Bosma's welfare remained their "greatest priority."
The 32-year-old father of one was reported missing in the Ancaster, Ont., area on Monday after going for a ride with two Toronto men who said they wanted to buy his truck, a black 2007 Dodge Ram 3500 diesel pickup with the Ontario licence plate 726 7ZW. Bosma had advertised the sale of his truck on the popular classified ad website Kijiji, as well as on AutoTrader.ca.
On Friday, police announced they had found Bosma's cellphone in an industrial area of Brantford. They also reported that Bosma's truck had been seen in downtown Brantford on Monday night.
Kinsella said Brantford police and Ontario Provincial Police were continuing to search an area near Oak Park Road, located just off Highway 403.
This is "a rapid, fluid, and dynamic investigation that is changing minute to minute," Kinsella said, adding that "information is being obtained, search warrants being prepared and evidence is being gathered."
"We are encouraged with the progress of the investigation and must be cautious in our approach over the next few critical hours."
Reacting to news of the arrest, Bosma's father-in-law Louis Veenstra told CBC News that since the charge was for confinement, not murder, the family is still hopeful Bosma is alive.
"Until I'm told otherwise, he's out there. He's alive," Veenstra said, describing the past few days as a "week of hell."
Bosma's brother-in-law, Chris Noordam, added the family only found out about the arrest through the media. The police are cautious of what information they share, he said, but the family is understanding.
"They don't want to jeapordize anything. You tell us too much and all of a sudden we get excited and we tell the wrong people," said Noordam, who is married to Bosma's sister.
"We know they have more information but they're withholding that for life and death. If they release too much right now, that could be the end of Tim."
A police officer himself with Sarnia Police Services, Noordam had some strong words for those involved in Bosma's disappearance.
"Turn yourself in," he said. "We know who you are. We know where you are. We will find you. There's no hope for you."
The driveway of Bosma's family home in Ancaster, where he was last seen, was lined with dozens of vehicles Saturday as friends and family gathered to comfort each other. His wife, Sharlene, is holding up as well as can be expected, her father said.
"Sharlene is doing amazingly well under the circumstances," he said. "It's not easy, the unknowns. It's hard."
He also told CBC News that the couple's two-year-old daughter is "in a safe place and she's doing well."
Earlier this week, Hamilton Det. Sgt. Matt Kavanagh said the two suspects in Bosma's disappearance test-drove a similar truck in Toronto but didn't harm the seller.
That seller, who was in the Etobicoke area, had also posted the truck for sale online. He told police he found the men suspicious, because they had arrived to see the truck in an industrial area with no vehicle.
"He thought it was odd, because of the size of the vehicle," Kavanagh said. "It's more of a business vehicle than a personal vehicle.
"Fortunately for this person, he's a very large individual. I think he would have overpowered the both of them. I think that was his advantage."
Kavanagh also stressed that at this point, there's nothing to indicate something from Bosma's past led to the disappearance.
He also announced that a tip line has been created. The public can call 905-546-2100 with any information.