The Austrian hitman has penned a three and a half year deal with the Portuguese giants after the two sides finally agreed a fee for the transfer to take place
The forward has been in talks with the Europa League holders since Sunday, after his agent Rob Jansen travelled to Portugal with the intention of thrashing out an agreement.
However, an undisclosed fee was eventually haggled along with the necessary documentation being signed and sealed, and Janko has spoke of his relief at seeing a move finally bear fruit.
"It was very strange for me when there was much [of the deal] still to be completed," Janko told Sport Promotion.
"Of course you assume that everything will be okay, but still, what if it does not happen? Now everything is done and I can therefore only be satisfied.
"Porto is of course a great club to play for. Not only in Portugal but also in Europe. They qualify for the Champions League almost all the time and I consider it an honour to represent this club.''
Janko also explained the reasons why Porto were in hot pursuit of his services, pinpointing the club's lack of a clinical finisher since the departure of Radamel Falcao to Atletico Madrid last summer.
"Goals, that is what is expected of me. The club was looking for a 'killer' in the penalty area, so they came to me," he continued. "I'll do everything possible to meet these expectations."
The Austrian made his name at Red Bull Salzburg in a five-year spell between 2005 and 2010, where he netted an impressive 75 goals in only 108 appearances, before earning a move to then-Dutch champions Twente.
Janko managed to sustain his goalscoring form in the Netherlands, reaching double figures in each of his two seasons in Enschede, including a haul of 17 to date this season.
"I've always done my best, trying to score as many as I could. I could not do more than that as a player.
"I'm looking not for resentment. On the contrary, I have had a wonderful time at Twente and appreciate the fans' support,'' the 28-year-old concluded.