3 May 2017 Last updated at 11:09 BST
It's part of a mega bridge called the Hong Kong-Zhuhai-Macao Bridge, which will connect three different places on the coast.
They used the world's heaviest crane ship to hoist the 6,000-tonne piece of concrete into place.
Jenny has all the details.
Engineers have been doing the tricky job of slotting in the last section of a huge undersea tunnel in south-east Asia.