Jurors on Monday also found Kouri Richins guilty of fraudulently claiming insurance benefits after the death of Eric Richins in March 2022 at their home outside the ski town of Park City.
Prosecutors say Kouri Richins slipped five times the lethal dose of the synthetic opioid into a cocktail the 39-year-old drank.
They say Richins was $US4.5 million ($A6.4 million) in debt and falsely believed that when her husband died, she would inherit his estate worth more than $US4 million ($A5.7 million).
They also say she was planning a future with another man she was seeing on the side.
After her husband's death, Richins self-published a children's book about grief to help her sons and other kids cope with the loss of a parent.
She was also convicted of other felony charges, including an attempted murder charge in what authorities say was another effort to poison her husband weeks earlier on Valentine's Day with a fentanyl-laced sandwich that made him break out in hives and black out.
Richins' defence lawyer said Eric Richins was addicted to painkillers and had asked his wife to procure opioids for him.
Kouri Richins, however, told police earlier that her husband had no history of illicit drug use.
"She wanted to leave Eric Richins but did not want to leave his money," said Summit County prosecutor Brad Bloodworth.
Richins pleaded not guilty to all charges.
The most serious charge - aggravated murder - carries a sentence of 25 years to life in prison.
What was scheduled to be a five-week trial was cut short last week when Richins waived her right to testify, and her legal team abruptly rested its case without calling any witnesses.
Richins' lawyers said they were confident that prosecutors did not produce enough evidence over the past three weeks to convict her of murder.
"They haven't done their job, and now they want you to make inferences based on paper-thin evidence. They want you to do their job for them.
Tell them, 'No,'" defence lawyer Wendy Lewis urged the jury.
Prosecutors said Richins, a real estate agent focused on flipping houses, was deep in debt and planning a future with another man she was having an affair with.
She had opened numerous life insurance policies on her husband without his knowledge, with benefits totalling about $US2 million ($A2.8 million).
They showed the jury text messages between Richins and Robert Josh Grossman, in which she fantasised about leaving her husband, gaining millions in a divorce and marrying Grossman.
The internet search history from Richins' phone included "what is a lethal.dose.of.fetanayl (sic)," "luxury prisons for the rich America" and "if someone is poisned (sic) what does it go down on the death certificate as," a digital forensic analyst testified.
Bloodworth replayed for the jury a clip of Richins' emergency call from the night of her husband's death.
That's "not 'the sound of a wife becoming a widow,'" he said, quoting the defence's opening statement.
"It's the sound of a wife becoming a black widow."
Shortly before her arrest in May 2023, Richins self-published the book Are You with Me?
Summit County Sheriff's detective Jeff O'Driscoll, the lead investigator on the case, testified that Richins paid a ghostwriting company to write the book.
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