The 980-acre North Lanarkshire park, which will be used as a venue during the 2014 Commonwealth Games, was a bigger draw than Edinburgh Castle and Glasgow's Kelvingrove Museum.
It sits between Hamilton and Motherwell and is home to a theme park, play facilities and an artificial loch.
The figures, published by VisitScotland, found that Glasgow's Riverside Museum and Kelvingrove Art Gallery and Museum both attracted more than a million visitors.
In Edinburgh, almost 1.9m people visited National Museum of Scotland and 1.2m passed through Edinburgh Castle.
A total of 1.1m people flocked to the shores of Loch Lomond and more than a million visited Dean Castle Country Park in Kilmarnock.
Scotland attracted 2.2m overseas visitors last year, generating a £1.4billion boost to its economy.
Most were from the US, with 324,000 spending a total of £274m. There were 281,000 visitors from Germany and 174,000 from France.
Glasgow and Clyde Valley was the second most popular area for international visitors, just behind Edinburgh and the Lothians.
VisitScotland chairman Mike Cantlay said: "The report is useful as it allows us to gain an insight into visitors, their activities and how they are likely to spend money."
North Lanarkshire Council's environmental services convener Helen McKenna said: "It's great news that Strathclyde Park is Scotland's top visitor attraction. It's the perfect place to spend time.
"As a world-class water sports facility, it will be showcased to a global audience as a venue for the Commonwealth Games triathlon."