Just minutes before the midnight ET deadline, Alonso and the Reds agreed to a Major League contract through 2012, reportedly worth $4.5 million plus incentives.
By signing a Major League deal, the Reds must place the 22-year-old first baseman on the 40-man roster, which was one of Alonso's primary demands.
"It was very important for me to be on the 40-man roster," he said. "Just being part of the club and having something to do with the club was big."
Alonso's close pal, Yankees slugger Alex Rodriguez, offered him advice throughout the negotiations. Alonso first met Rodriguez when he was 9 years old, and A-Rod has served as his friend and mentor ever since.
"We talk every week," Alonso said. "He gives me a lot of advice. He told me to stay relaxed, not to panic. He told me that eventually we'll get something done. If it doesn't work out, at the end of the day, I was going to play baseball. It's an honor to know him."
Rodriguez offered to allow Alonso to live with him in the event he signed with the Long Island club.
Instead, Alonso is scheduled to report to Sarasota, Fla., on Monday to work out. He's expected to join the Reds' Class A affiliate in Sarasota shortly thereafter.
"This makes us all happy," Reds manager Dusty Baker said of Alonso's signing. "I got a call from [Reds general manager] Walt [Jocketty] on my home phone. I had my cell phone turned off. When my phone rang, I knew it was going to be good news or bad news."
Alonso, who hit .370 with 24 home runs and 72 RBIs as a junior at Miami, believes he could play first base, third base or left field in the Major Leagues.
He's also not lacking for confidence.
"I play with a lot of hunger," Alonso said. "I consider myself a line-drive hitter. I can drive the ball, but I'm more of a gap-to-gap guy. I'm a very passionate guy. I take baseball very seriously. I'm a winner."