"I am at a loss, upset, hurt and have to speak out,” he wrote in the message. “It’s affecting my life, mental state and is a daily concern."
"My costars, friends, family, myself, in fact I’m associated with, has been subjected to personal slurs, shaming, abuse, death threats, stalking, sharing private information and vile, false narrative," he said.
The Scottish actor said he has been falsely accused of "manipulating fans, being a closet-homosexual, trying to mislead or exhort fans for money and disregarding Covid advice."
He did not name his bullies but implied they were people writing on blogs and social media.
Heughan also said he and his castmates had been stalked and harassed and that there had been attempts to hack their emails.
He called the bullies "adults who should know better" and said he was in the middle of legal action.
"I'm so hurt by this," he said.
The criticism about Heughan's COVID-19 response stems from the fact that the actor, who is a citizen of the United Kingdom, has been posting updates from Hawaii while riding out the coronavirus pandemic there.
"I came here before the travel ban. Non [sic] of us knew how bad things would get," he added. "Upon the advice of everyone I trust, I decided to remain in a safe environment. It was a good decision. I'm safe, isolated, putting no one at risk and am not a burden to the locals."
Within hours of the post, #WeStandWithSam started trending on Twitter as several of the actor's costars came to his defense.
Outlander, which is based on a book series by author Diana Gabaldon, is known for having an extremely dedicated fanbase. Many of those fans have tweeted and asked for people to treat Heughan with kindness.
Heughan, who is known for engaging with his fans, ended his statement by thanking supporters and telling detractors to just unfollow him.
“As an actor in these times, we feel impotent," wrote the 39-year-old. "We can’t do much but I have tried to use what leverage I have to provide a voice to charities that need it and hopefully a little entertainment or light relief. For those still unhappy I suggest you unfollow.
"To every fan that has supported me and the work I do, THANK YOU," he concluded. “I’m so grateful, from the bottom of my heart. Stay safe and please be kind to yourself and each other.”