The Parkland gunman who allegedly shot and killed 17 people registered to vote while behind bars — a move which the dad of one of the slain teens blasted as “despicable,” according to reports.

Nikolas Cruz, 20, registered as a Republican on July 25 while locked up at Broward County Jail, the Sun-Sentinel of Fort Lauderdale reported.

Andrew Pollack, whose daughter Meadow was among those gunned down in the Valentine’s Day massacre, said he was “sick to my stomach” over the revelation.

“[He] murdered 17 students & staff, including my daughter Meadow,” Pollack tweeted Saturday. “Yet in July, Broward Sheriff @ScottJIsrael let people into the jail to get him & other animals registered to vote.”

In another post, he wrote, “My daughter was murdered and buried. She can’t vote.” Michael Udine, the commissioner of Broward County, also slammed Cruz’s registration as “outrageous and so hurtful” to the families of the victims.

“Regardless of the legality, it doesn’t negate the pain and the hurt when the people of northwest Broward who were affected by this monster see something like this,” he said.

Cruz, who is charged with 17 counts of murder and 17 counts of attempted murder for the killing spree inside Marjory Stoneman Douglas High School in Parkland, is eligible to vote this year because he hasn’t been convicted of a crime.

Sheriff’s spokeswoman Veda Coleman-Wright said registering inmates is nothing out of the ordinary.

“Taking away the voting rights of an eligible person awaiting trial is illegal,” she said.

Absentee ballots are sent directly to the jail and then returned to the supervisor of elections office to be counted.