The lawyer for the Massachusetts man accused of sending threatening letters containing white powder to five people — including one to Donald Trump Jr. that sickened his wife — said his client will plead not guilty.

Attorney Scott Gleason said Daniel Frisiello is “an overwhelmed young man.”

A federal judge in Worcester, Mass., ordered Frisiello held Thursday afternoon pending a detention hearing on Monday.

The 24-year-old Beverly resident allegedly sent the letters last month, and one arrived at Trump Jr.’s mother-in-law’s Upper East Side pad on Feb. 12.

The first son’s wife Vanessa started coughing and became nauseous when she opened it, and was briefly hospitalized.

The substance turned out to be harmless, authorities said.

Gleason said Frisiello had some “issues” and “difficulties” and had been on medication his entire life, but declined to elaborate.

Frisiello was suspended from his job at Catholic Charities of Boston’s Peabody child care center.

He was arrested Thursday morning when the feds raided his home.

Trump Jr. later responded on Twitter.

“My family and I are incredibly relieved to hear this news. No one should ever have to deal with this kind of sickening behavior,” he wrote, adding praise for the feds for cracking the case so quickly.