The Hindu nationalist party allied with the regional People's Democratic Party (PDP) in 2016 to rule the Muslim-majority state.
The state elections were being seen as a marker of the popularity of Prime Minister Narendra Modi and the BJP as well as the opposition Congress party ahead of general elections next year.
A win in Karnataka would be regarded as a major boost for the BJP and Prime Minister Narendra Modi ahead of general elections scheduled in 2019.
Both Narendra Modi and Congress leader Rahul Gandhi have campaigned hard in Karnataka, a state of 66 million people with diverse constituencies.
India's ruling party will make gains in a key election to rule the southern state of Karnataka but fall short of a majority, an opinion poll showed.
India's Supreme Court on Tuesday rejected bids by two states to re-instate a ban on the release of controversial Bollywood film Padmaavat.
The rejig of the council of ministers, which also saw the exit of six members, is being seen as an attempt by Modi to improve the performance of his government as the 2019 general elections near.
An Indian court on Thursday sent the son of a leading politician to police custody for harassing and trying to abduct a woman, in a case that has embarrassed the ruling party as it runs a campaign on women's safety.
Indian Prime Minister Narendra Modi's party has failed in its bid to oust the opposition Congress party's top strategist from the upper house of parliament, in a rare setback to its effort to secure national political supremacy.