A sub-inspector (SI) is generally in command of few police personnel (with head constables, the equivalent of corporals, commanding police outposts). He is the lowest ranked officer who under Indian Police rules and regulations can file a charge sheet in court, and is usually the first investigating officer. Officers subordinate to him cannot file charge sheets, but can only investigate cases on his behalf.

A sub-inspector ranks above an assistant sub-inspector of police and below an inspector (who usually supervises a group of police stations). Most sub-inspectors are directly recruited into the police and have better educational qualifications than lower-ranking police officers. In busy police stations there are also additional sub-inspectors (Addl. SI), who are generally officers who have come up through the ranks. In this case, the directly recruited sub-inspector in charge is referred to as the principal sub-inspector.

Specialised units such as the Central Armed Reserve Police Force, State Armed Reserve Police and Armed Battalions use the same rank, but generally these officers do not have any investigative powers. There are also specialist non-investigative officers in other Indian law enforcement agencies, such as sub-inspector (band) and sub-inspector (motor transport).

The rank insignia for a sub-inspector is two (five point) stars, and a red and blue striped ribbon at the outer end of the shoulder straps. This is similar to the insignia of a subedar in the Indian Army. The assistant sub-inspector will have one (five point) star, and a red and blue striped ribbon at the outer end of the shoulder straps. This is similar to the insignia of a naib subedar in the Indian Army. The rank insignia for principal sub-inspector andadditional sub-inspector are one and the same.