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Ronnie Ortiz-Magro just pled not guilty to a laundry list of misdemeanors stemming from last month's incident with his baby mama, Jen Harley.

The "Jersey Shore" star showed up in an L.A. courtroom Friday morning, sporting a suit and tie for his arraignment, and entered his not guilty plea to the 7 misdemeanors he's facing -- domestic violence, child endangerment, brandishing a weapon, criminal threats, false imprisonment [of Jen], and 2 counts of resisting arrest.

As we reported ... Ortiz-Magro was initially busted for kidnapping after allegedly striking Jen, chasing her down with a knife, taking their baby girl from her ... and then locking himself inside an Airbnb they were renting in Los Angeles.

When police arrived, he allegedly refused to come out, so cops busted in and tased him before slapping a pair of handcuffs on him.

Jen says she suffered bruises and scrapes from her altercation with Ronnie, and there are photos of her alleged injuries.

TMZ broke the story ... Jen was granted a protective order to keep Ronnie away from her and their 1-year-old daughter, Ariana. We're told he plans to fight that order during a court hearing next week.

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You'll recall ... neighbor's surveillance footage shows Jen looking very scared as she held Ariana and hid behind a car.

As we've reported ... Ronnie denies wielding a blade during the incident or injuring Jen, and he claims he was only trying to protect their child.