Nirmala Sitharaman's appointment in a cabinet reshuffle will ease the burden on Arun Jaitley, who had been juggling finance and defence throughout a months-long impasse with China over a strategic Himalayan plateau.
Modi also dropped Suresh Prabhu as railways minister, less than a week after he offered to resign following a train accident in August that killed 10.
India PM plans cabinet revamp, some ministers offer to quit, sources
Nine junior ministers were sworn in and four promoted to the cabinet in the reshuffle, the third shake-up since Modi was elected in 2014.
"I congratulate all those who have taken oath today. Their experience and wisdom will add immense value to the Council of Ministers," Modi said after the swearing-in ceremony at the presidential palace.
Sitharaman, a junior minister who oversaw the commerce and trade portfolio, joins five other women in India's cabinet. The prestigious foreign affairs portfolio is also held by a woman, Sushma Swaraj.
Prime Minister Indira Gandhi also acted as defence minister on two occasions between the mid-1970s and early 1980s. She was assassinated in 1984.
Sitharaman's appointment comes just days after India and China agreed to end a months-long military stand-off at a strategically important disputed area in the Himalayas.
New Delhi said both sides agreed to withdraw troops from an area near the Indian border that is claimed by both China and India's ally Bhutan.
Modi urges India to reject violence in name of religion
The latest appointment will allow Jaitley to focus on his finance portfolio at a time when India's economic growth has slowed to its weakest pace in three years.
Piyush Goyal, the former energy minister, was promoted to cabinet after being assigned the railways portfolio, overseeing 1.3 million employees.
Modi has made overhauling India's creaking railway network a key priority, and speculation was rife that Prabhu would be replaced following a series of accidents.
Prabhu offered his resignation in late August after an accident earlier in the month killed 10. A similar disaster in December claimed 146 lives.
He will assume Sitharaman's former portfolios at commerce and trade as a junior minister.
The reshuffle has been cast as Modi laying the groundwork before national elections in 2019, where he is widely tipped to defeat a diminished opposition.
His nationally ruling party also governs 18 of India's 29 states, either directly or in alliance with regional parties.
COMMENTS (2)
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ