The 23-year-old taekwondo fighter scored 8.690 to pip Iran's Marjan Salahshouri (8.470) to the gold in women's individual poomsae event, triggering wild celebration at the Plenary Hall of the Jakarta Convention Centre.
Her head covered in a hijab and the national flag draped around her shoulders, an ecstatic Rosmaniar ran around the arena before being escorted to the VIP gallery where Indonesia President Joko Widodo congratulated her.
Rosmaniar's victory earned Indonesia their first ever taekwondo gold in the Asian Games, which they are hosting for the second time.
She also trended on Twitter with local celebrities congratulating her while fans ran hashtags such as #terimakasihdefia, meaning 'Thank You Defia'.
Leading the praise was Widodo, whose motorbike stunt in Saturday night's opening ceremony created a similar buzz on social media.
"Defia, the people of Indonesia are proud of you!" Widodo tweeted.
Rosmaniar's victory was a boost for Indonesia who have set themselves a target of 16 gold medals at the Games.
"I feel really happy and proud. And I would like to thank my fellow Indonesians for supporting me," Rosmaniar responded on her own Twitter feed, adding some encouragement for her team mates: "Keep the spirit up, you all can be winners."
Beaten semi-finalists Jihye Yun of South Korea and Malaysian Yap Khim Wen won a bronze each.
South Korea, who have dominated the event at the Asian Games since its 1986 inclusion, have won two golds, one silver and one bronze in individual and team poomsae events so far.
(Additional reporting by Tabita Diela in Jakarta; editing by Christian Radnedge)
