According to Official Guinness Records,
The smallest functioning mousetrap (replica) measures 8mm x 4mm (0.31 x 0.15 inches), and was constructed by Talabathula Sai (India), in Andhra Pradesh, India, as verified on 2 Janury 2017. The mouse trap is made of wood and metal and functions as a mousetrap would.
For a complete list of 2016 records, please visit 2016 Guinness Records in India.