Adam Kirby scored 141 in the IQ test that reportedly ranked him smarter than US President Barack Obama and British Premier David Cameron.
The toddler was invited to join the elite group after demonstrating his ability to read Shakespeare and understand Japanese, Spanish and French, the 'Daily Express' reported.
The toddler stunned his parents Dean and Kerry-Ann Kirby by spelling 100 words and mastering both his times tables and the periodic table of chemical elements.
"We used to show him cards with the words hippopotamus and rhinoceros on them and he could identify the right animals most of the time," said Dean.
More From This Section
Adam's IQ of 141 - just four shy of the Genius category - compares with the average Briton's score of 100.
At two years five months, Adam is the youngest boy to join Mensa, the report said.
Only Elise Tan-Roberts, from London, who is now six, was younger, joining at two years four months.