The ex-NFL star has been hit with paternity docs by a 35-year-old woman who claims 45-year-old Key knocked her up back in 2016 -- and she's demanding child support for her 4-month-old son.

But Key says the allegations are 100% false.

According to court docs obtained by TMZ Sports, the woman has demanded a paternity test to establish Keyshawn as the father -- and once it becomes official, she wants at least $4,346 per month in support.

The rub ... Keyshawn is still legally married to another woman, Jennifer Conrad, and was at the time of the alleged conception.

Interesting note ... Jennifer had initially filed for divorce in March 2015 -- but they reconciled ... until August 2016 when she told TMZ Sports the divorce was back on for good.

At the time, it was unclear why the reconciliation went south.

We spoke with Key's attorney, Aja Mann, who tells us, "The allegations are absolutely false."