Police had described 18-year-old Sirsa as the main shooter in the murder of Moose Wala. He is a member of the Lawrence Bishnoi gang, police said.
Police arrested Sirsa on Sunday night from a bus terminal in Delhi. According to them, the 18-year-old was the closest of the shooters and fired six bullets at Moose Wala.
Meanwhile, an associate of his, Sachin Virmani has also been taken into custody.
It may be noted that Punjabi rapper and singer Sidhu Moose Wala, aged 28, was shot dead on May 29 by a group of assailants in Punjab. He died within 15 minutes of being shot.
Doctors recovered as many as 19 slugs from his body. The main accused in the case is gangster Lawrence Bishnoi, who police believe, planned the murder from jail.
Officials informed that the teenager Sirsa went closest to the singer, who was in an SUV, and fired at him with both hands. He was the "most desperate" of the lot, the police said.
Investigations have revealed that Sirsa had received instructions to carry out the killing of Moose Wala a day before from Canada based gangster Goldy Brar, an aide of Lawrence Bishnoi.
Brar had claimed responsibility for Moose Wala’s killing via a Facebook post soon after the incident.
Earlier, the Delhi police arrested three accused in the case from Kutch in Gujarat. They were Priyavrat alias Fauji (26), a resident of Sonipat in Haryana, Kashish (24), also from the state's Jhajjar district, and Keshav Kumar (29), a resident of Bhatinda in Punjab.