The landmark decision should make access easier for people, although buyers will still need to have a consultation with the pharmacist.
The reclassification applies to two brands of "progesterone only" pill - Hana and Lovima.
Combined oestrogen and progesterone pills still require a prescription.
The move has been welcomed by campaigners who say the coronavirus pandemic has made a difficult situation even worse for those trying to access contraception services.
Sexual health doctors say services have been stretched and underfunded for some time.
Hana and Lovima, which contain 75 micrograms of desogestrel, are still available for free on prescription too.
They should be taken once a day, with no break between packs. If a dose is missed by more than 12 hours, the contraceptive effect may fail.
Taken correctly, the mini pill is more than 99% effective, meaning fewer than one in 100 women using it will get pregnant in a year. In reality, because of a range of factors, it is about 91% effective.
Most women can take desogestrel even if they are over 35 and smoke, unlike the combined pill, where risks can outweigh benefits.
* Confidential contraception services are free on the NHS, including for under-16s
* There are lots of types to choose from, but condoms are the only type that protect against sexually-transmitted infections as well as pregnancy
Dr June Raine, head of the Medicines and Healthcare products Regulatory Agency, said: "This is good news for women and families.
"We have consulted a wide range of people to enable us to reach the decision to make this contraceptive available for the first time in the UK without prescription. We received many responses to our consultation, the majority of which supported this approach."
The Royal College of Obstetricians and Gynaecologists said it was delighted by the news.
Dr Asha Kasliwal, president of the Faculty of Sexual and Reproductive Healthcare, said: "Progestogen-only contraceptive pills are safe, reliable, easy to use and are an incredibly popular contraceptive method. Availability over the counter in pharmacies will make it easier for women to access essential contraception to avoid unplanned pregnancies during and beyond Covid-19."
She said successive cuts to public health budgets have made it harder for women to get the contraception they need.
"Reclassification may also reduce unnecessary pressures on GPs, who will not need to see patients for repeat prescriptions.
"However, reclassification of one brand is only the first step, and buying contraception should definitely not be the only solution. We are calling for these pills to be available to everyone for free in community pharmacies, as well as the reclassification of other contraceptives moving forward."