The force initially decided that no offence had occurred on 30 April last year, but said it had since received "significant new information".
It added that it had delayed announcing the investigation until after Thursday's local elections.
Sir Keir said he was confident he hadn't broken any Covid rules.
He has faced criticism since he was filmed drinking a bottle of beer while in the constituency office of City of Durham MP Mary Foy.
Speaking to reporters on Friday, he said he had "stopped for something to eat" during meetings, and there was "no party".
"The police obviously have go their job to do - we should let them get on with it," he added.
A Durham Constabulary spokesperson said an "assessment" of whether lockdown rule breach had happened was carried out earlier this year.
They added: "At that time, it was concluded that no offence had been established and therefore no further action would be taken.
"Following the receipt of significant new information over recent days, Durham Constabulary has reviewed that position and now, following the conclusion of the pre-election period, we can confirm that an investigation into potential breaches of Covid-19 regulations relating to this gathering is now being conducted."
The Durham Constabulary investigation comes after Boris Johnson, his wife Carrie, and Chancellor Rishi Sunak were fined by the Metropolitan Police for attending a birthday party for the PM in Downing Street as part of an investigation into 12 gatherings across government buildings during the pandemic.
On Wednesday, Sir Keir said he had not been contacted by police over the gathering in Durham, adding that voters were "just about fed up with the mudslinging going on" regarding his behaviour.
In response to the news of the Durham Constabulary investigation into the Labour leader, shadow cabinet minister Emily Thornberry told the BBC: "We will answer any questions that the police have got for us and we are completely confident that no rules were broken, so it's fine."
She added that the "context" of the gathering in Durham had been "totally different" to that involving Mr Johnson and Mr Sunak in Downing Street.
Ms Thornberry also said that that the chancellor "did not go" once he realised an "organised" party was going on.
Richard Holden, Tory MP for North West Durham, wrote to Durham police last week asking them to reconsider an investigation into Sir Keir, who has called on Mr Johnson to resign over his behaviour.
It had been assumed that the recent calls for a new probe - which blared from the front pages of Conservative-supporting papers - would be somewhat muted after polling day.
But the re-opening of the so-called Beergate investigation will, in the short term, be uncomfortable for Sir Keir, and could yet have long-term consequences.
A further diminution of trust in politics, rather than specifically in the prime minister, who was fined for breaking lockdown rules, isn't the outcome Labour would want.
The bigger fear, though, must be the possibility that the opposition leader gets a fine, a Fixed Penalty Notice.
That would take the sting out of the forthcoming Sue Gray report into Partygate.
But as Sir Keir based much of his attack on contrasting his integrity with prime ministerial rule-breaking, and has called for Boris Johnson to go, then it would seem highly likely that an FPN could spell the end of Sir Keir's Labour leadership.
If he genuinely believes he has done nothing wrong, this former director of public prosecutions would then have to weigh up whether to challenge it in court.
Of course, Labour officials will tell us not to get carried away.
They say they are happy to co-operate with the police, and are completely confident that no rules were broken.
So it's possible that Sir Keir is cleared before the Sue Gray report is published, and the focus will return again to the prime minister's own behaviour.
But today's developments in Durham will lead to some nervous moments in the Labour hierarchy, and will be far from welcome when they had wanted to trumpet local authority victories.
England was in the "Step 2" rules, which had been introduced on 12 April.
Gathering indoors with people from outside your household or support bubble was against the law.
There was an exemption for "work purposes", although working from home was recommended in the guidance, but the rules did not mention socialising at work.
The question for police would have been whether Labour officials eating and drinking together would have been "reasonably necessary for work" and whether drinking beer makes a difference to that.
Sir Keir Starmer has told LBC Radio that, as pubs and restaurants were closed and his hotel was not serving food, "if you didn't get a takeaway then our team wasn't eating that evening".
Bars, pubs and restaurants were allowed to open outdoors for groups of six people or two households, but indoor service was not allowed.