The centre-back who featured in 66 Italian top-flight games having previously moved from Turkish Super Lig outfit Bursaspor, is the latest addition to the Vicarage Stadium outfit - and penned a five-year deal with Vladimir Ivic’s team for an undisclosed amount.
Born and raised in England, the 27-year-old started his professional career in the Dutch elite division with Groningen and Dordrecht before heading to Gent and Haugesund respectively.
He joins compatriots Tom Dele-Bashiru and Isaac Success in Hertfordshire and has been handed the No. 5 jersey.
Troost-Ekong is expected to play a crucial role in the team’s return to the Premier League.
Speaking on his move to England, the defender attributed the transfer to his ambition of featuring in the English elite division.
"I believe in the project for Watford to get back into the Premier League," Troost-Ekong told BBC Sport Africa.
"It's always been my dream to play in the Premier League and Watford has given me the platform now to get there. Looking at the squad, I believe we have the quality to do it.
"I've always made tactical decisions for my career and this felt like the right step now for me and my family.
"As for my international future, I have shown what I can add to the Super Eagles. I have consistently contributed to the team's success in the last five years and have had the pleasure of captaining the team."
Troost-Ekong also played a sterling role in Nigeria U23’s third-place finish at the 2016 Olympics staged in Rio. In addition, he featured for the Super Eagles in the 2018 Fifa World Cup in Russia as well as the 2019 Africa Cup of Nations hosted by Egypt.
Watford also confirmed the arrival of Guinea youth international Djibril Toure. The 17-year-old, however, will join the club on January 1, 2021 from Ceffomig FC.