The Prison Lions FC - a team consisting of inmates and prison officers - has played several friendly matches but this season marks their league debut.
David White, the physical educator at Guernsey Prison, said: "They're really buzzing about it. It's a good vibe for the prison."
All the team's 15 matches - 'home' and 'away' - will be played at the prison.
Mr White said participation would be on a reward basis, meaning the league would provide additional incentive for some prisoners to display good behaviour.
"They love their football and it's their 90 minutes of freedom," he said.
He said his side would be aiming to win the six team league and he was looking forward to seeing prisoners and officers playing together, as opposed to against each other, as they do for an annual charity match.
Rod Hamon, the president of the Sunday Soccer League, said some of the rules had had to be bent in order to accommodate the prison team.
Registration of new players is not normally permitted after 31 December but Mr Hamon said: "If players are released after Christmas, they might be left with no squad, so we've got to make a few allowances."
Mr White said visiting teams would need to follow all the prison's usual security procedures, meaning they would be searched as they arrived for the game.
The Prison Lions were granted entry into the league by a unanimous vote of delegates from the other teams.
Mr Hamon, who is also a referee in the league, said they were "delighted" to welcome them and said there had been no concerns expressed about any behavioural issues.
"If they misbehave, they've got more to lose than the people coming in from outside and to be honest, we get more stick from the players outside than from the prisoners," he said.
Guernsey's Les Nicolles Prison has had its football team entered into the island's Sunday Soccer League.