Former Porto, Chelsea and Tottenham boss Villas-Boas was unveiled to the media on Thursday, following Zenit's exit from the UEFA Champions League a day earlier.
The Russian Premier League giants defeated Borussia Dortmund 2-1 in Germany on Wednesday but were still eliminated on aggregate 5-4.
It was a spirited display though, and Villas-Boas – who will be immediately tasked with winning the league – says it was an easy decision for him to make.
He told a news conference: "I've been very well received by all the people I've met in St. Petersburg.
"To be honest, there was no difficulty in taking this decision for me, because Zenit is a well-known club in Europe.
"The ideas that led to this decision suit my own ideas about football. This is an important step for me, that suits my ideas, and my career.
"Of course I knew a lot of Zenit's players before I came here. I'll learn more about them in the process of my work."
Villas-Boas confirmed that interim coach Sergey Semak would remain at the club, as part of the first-team staff.
Zenit have won just one of their last seven league matches, form that saw former coach Luciano Spalletti sacked.
They have dropped to second in the table but encouragingly, are just three points behind leaders Lokomotiv Moscow.
Spartak Moscow, Dinamo Moscow and CSKA Moscow are all within four points of Zenit though, meaning the race for the title will be hotly contested.
"If we win all nine remaining matches, Zenit will be champion," Villas-Boas added.
"My goal is to bring Zenit to the top."
The 36-year-old also said he was pleased with the signs he saw from Zenit in Wednesday's triumph at Signal-Iduna Park.
"Last night's game was an excellent result for sure," he continued.
"The main thing is that Zenit played as a team. I saw the team's spirit."
