Cllr Nick Worth, executive member for culture and heritage at Lincolnshire County Council, said: "I don't think anyone expected us to reach this point quite so quickly.
"We've worked hard to make The Collection one of the best museums in the country, and thisimpressive milestone shows that those efforts are paying off.
"We're always looking for ways to improve what's on offer for visitors, making sure we have something for everyone.
"We've pieces by well-known masters, innovative and award-winning work from well-respected contemporary artists, and historical exhibitions that bring the story of our county to life like never before.
"So if you're one of the few who hasn't discovered everything this fantastic site has to offer, make sure you pay us a visit soon."