Artur Boruc hopes to become Poland's number one
A fee had been agreed between the clubs and Celtic have the option of paying more to extend the 25-year-old's stay.
Boruc hopes that joining Celtic will help him take over the Poland jersey from Liverpool's Jerzy Dudek.
"I want to be number one with Celtic and with Poland and win myself a longer contract here," he told BBC Sport.
"If I play over here then Jerzy Dudek will have a little problem over in Liverpool."
Boruc, who will contest the number one jersey at Celtic with Scotland international David Marshall, was also delighted to be given the chance to play in front of regular 60,000-strong crowds.
We felt we saved a lot of money and we can use that elsewhere to strengthen the side
Celtic manager Gordon Strachan
The Pole had promised not to "ask for a lot of money" in negotiations as he was simply honoured to be joining a club like Celtic.
"This is a big club and a great club and maybe we are going to be in the Champions League," he said.
"I remember seeing one Celtic game on television - in Seville against Porto - and the one thing I remember was that there were so many supporters."
Boruc will be linking up with fellow Pole Maciej Zurawski, who completed his move from Wisla Krakow last week.
"For me, the acclimatisation should be easier at a new club because I can speak English a bit," added Boruc.
"But I will have a Polish colleague too, which is very important."
Celtic turned their attention to Boruc after failing in a bid for Southampton's former Rangers goalkeeper, Antti Niemi.
The Glasgow club's manager, Gordon Strachan, said: "We tried to get Antti Niemi and that deal came to an end in the early hours of Friday morning, when we made our final bid.
"What we are getting is a player who is hungry. He's the Polish number two at the moment.
"There is a loan fee and his wages, but we felt we saved a lot of money and we can use that elsewhere to strengthen the side."
Celtic chief executive Peter Lawwell defended the decision to take Boruc on a similar deal to that which led Craig Bellamy to star for Celtic on loan from Newcastle but fail to conclude a permanent move.
"Many deals are done on that basis in the footballing world, particularly in Scotland," he said.
"We have agreed a fee so if things do work out then we will take that option."
600 lub 650 tysięcy euro dostanie Legia Warszawa za roczne wypożyczenie bramkarza Reprezentacji Polski, Artura Boruca do zespołu wicemistrza Szkocji, Celticu Glasgow. Drużyna z Wysp Brytyjskich będzie miała również prawo pierwokupu zawodnika, który rywalizuje z Jerzym Dudkiem o miejsce w bramce polskiego zespołu narodowego.

Bramkarz wyrazi nadzieję na szybkie wywalczenie sobie miejsca w pierwszym składzie drużyny z największego szkockiego miasta. Dodał też, że postara się zasłużyć sobie na miejsce w pierwszym składzie zespołu.

Boruc dołączy do byłego piłkarza Wisły Kraków, Macieja Żurawskiego, który przed paroma drużynami dołączył do szkockiej drużyny. Spekuluje się, że Celtowie są zainteresowani także Markiem Saganowskim z Legii Warszawa i Kamilem Kosowskim, wypożyczonym z Wisły Kraków do niemieckiego FC Kaiserlautern.

Wypożyczenie Boruca wyklucza możliwość pozyskania przez Celtic fina, Artura Niemi z Southampton.