MLB's Carlos Martinez Calls BS on STD Lawsuit ... '100% False'

St. Louis Cardinals pitcher Carlos Martinez is ferociously disputing a lawsuit -- in which the 24-year-old is accused of giving a woman non-life threatening STDs ... after swearing he was clean.

The woman behind the suit, filed in district court in Miami, claims she began a romantic relationship with Martinez in 2012 -- and the two had an on-and-off relationship for years.

According to the suit, the two reconnected last year -- and began having sex again on Dec. 16th. Two days later, the woman claims she began to feel ill and later tested positive for multiple STDs.

The woman claims she had grilled Martinez about his sexual health before they hooked up and he "swore to God that he was not infected with any sexually transmitted disease."

Soon after the woman informed Martinez about her condition, he allegedly bolted for the Dominican Republic and never called her back.

Now, she's suing him for battery, negligent transmission of sexually transmitted diseases, fraud and more ... and she's demanding more than $1.5 million in damages.

We spoke with Martinez's attorney Ruben Scolavino who tells us Carlos adamantly denies the allegations.