Michelle Carter, now 20, will serve 15 months of her term behind bars and five years of probation.
Carter was convicted in June of the involuntary manslaughter of Conrad Roy, who took his own life on 13 July 2014.
Mr Roy’s father told Thursday’s hearing: “My family is heartbroken. My son was my best friend.”
Carter had faced a maximum sentence of 20 years, but her legal team argued that both she and Mr Roy suffered from mental illness.
Her lawyer said it was a “horrible circumstance that she completely regrets”.
However, Judge Lawrence Moniz said he did not find that her “age or level of maturity or even her mental illness have any significant impact on her actions”.
A prosecutor read a statement from Mr Roy’s mother, Lynn Roy, saying there was “not one day I do not mourn the loss of my beloved son”.
“I want him to be proud of me and how I am handling everything – I am trying to be there for his sisters in all of my pain we will carry with us for eternity,” it said.
Mr Roy’s sister, Camden Roy, testified that she was “haunted” by the realisation that she would never see her brother get married or be an aunt to his children.
The case appears to set a legal precedent, as there is no Massachusetts law that criminalises telling a person to commit suicide.
Carter was 17 when Mr Roy was found dead of carbon monoxide poisoning in a vehicle in 2014.
The case drew national attention after Carter’s text messages revealed she had encouraged him to end his life.
“Hang yourself, jump off a building, stab yourself I don’t know there’s a lot of ways,” she said in several messages sent in the two weeks before his death while he was on holiday with his family.
In the moments before his suicide, she wrote: “You need to do it, Conrad” and “All you have to do is turn the generator on and you will be free and happy.”
Judge Moniz said Carter sought attention through Mr Roy’s death, ordering that she not be allowed to profit from the case through any press.