Venus is spinning more slowly and astronomers don't know why.
In the 1990s, they worked out one Venusian day - the time for the planet to spin round once - lasted 243.018 Earth days.
But now, the European Space Agency's Venus Express orbiter shows Venus' spin is getting even slower and a day on Venus is now six and a half minutes longer.
Scientists aren't sure why.
One cause could be Venus' thick atmosphere and high winds pushing against the planet's spin.
The news is important for future missions to the planet. Scientists use this data to plan missions to the planet and choose the best spot to land a rover.
The new speed means some features on Venus will be 20 kilometres away from they were expected to be.