The Janata Dal (United) president also took a dig at the Congress, saying that while the party was aiming to defeat the BJP in Gujarat, it "ended up losing even in Himachal Pradesh", where it was in power.
"Congratulations to honourable Prime Minister Narendra Modi and the Bharatiya Janata Party for the victory in the Gujarat and Himachal Pradesh Assembly elections. The Congress, which had claimed it would wrest power from the BJP in Gujarat, has ended up losing even Himachal Pradesh," Kumar tweeted in Hindi.
An official release issued said the chief minister spoke to Modi and BJP president Amit Shah in the morning over phone and extended his greetings on the poll victory to them.
Once considered a potential political rival of Modi, Kumar now maintains a cordial relationship with the prime minister, following the formation of a JD(U)-BJP coalition government in Bihar in July.
The Grand Alliance, comprising the RJD, JD(U) and Congress, had come to power in Bihar after the 2015 Assembly polls. The anti-BJP coalition had, however, collapsed over corruption charges surfaced against the then deputy chief minister Tejashwi Yadav, the son of RJD chief Lalu Prasad.
The reunion of the JD(U) and the BJP came four years after Kumar had severed ties with the saffron party in June, 2013 over the elevation of Modi as the NDAs prime ministerial candidate in the 2014 Lok Sabha polls.