All applicants to police forces in England and Wales will be asked from October to state their sexuality by ticking a box in the anonymous equal opportunities section of their application form.
They will be asked if they are heterosexual, bisexual, homosexual, lesbian or if they would prefer not to say.
All information will remain confidential, and it is not thought that the section will be obligatory.
John Gillic, a human resources manager with the county force and chairman of SPECTRUM, the support group for gay, lesbian and bisexual officers and staff working at Northamptonshire Police, said: "Sexual orientation is an invisible quality, but is as important a distinguishing characteristic as gender or race.
"Colleagues from minority groups have an absolute right to be treated with dignity, feel valued and safe from harassment and discrimination and therefore SPECTRUM support the principle of monitoring sexual orientation.
"Monitoring is necessary to establish a business case for positive action and to provide a benchmark against which to compare a range of other statistics such as complaints, assaults and unfair treatment.
"SPECTRUM looks forward to working with the force to introduce a system that continues to preserve confidentiality and gives everyone the opportunity to 'prefer not to say'."
The Gay Police Association said it supported the move as the police force needed to get an accurate picture of the number of gay officers serving in it.
Strategic director Sian Lockley said the difficulty of quantifying levels in the past had led to the marginalisation of some people.
She said: "If we already do gender and race monitoring, how do you defend not doing sexual orientation monitoring? There is no difference.
"It's not about asking people who they have slept with and it's not about quotas; we do not want a certain percentage of the workforce to be gay."
daniel.owens@northantsnews.co.uk