The news emerged days after she flew to London for medical tests. Her treatment begins immediately, her daughter said.
Ms Nawaz is due to stand for election to parliament in a by-election in her husband's former seat next month.
He was forced to resign in July after the Supreme Court disqualified him from office for not disclosing assets. Mr Sharif denies wrongdoing.
The couple's daughter, Maryam, tweeted that the cancer had been found on the left side of her mother's neck.
End of Twitter post  by @MaryamNSharif
In another tweet she said doctors were hopeful of curing Ms Nawaz because the disease had been diagnosed early.
Lymphoma is a cancer of the lymphatic system. It occurs when tumours build up in tissues found in areas such as the bone marrow or lymph glands.
Earlier reports suggested Ms Nawaz had throat cancer.
Maryam Sharif is now expected to run her mother's political campaign while she undergoes treatment.
"We hope that she will recover soon enough, but it is likely that she won't be around during the days leading up to the election," senior PML-N party member Mushahidullah Khan told BBC Urdu.
Mr Sharif had to vacate his seat after becoming embroiled in the Panama papers scandal.
The papers revealed his children were linked to offshore companies that owned properties in London.
Kulsoom Nawaz's decision to stand for election was seen as a change of plan for the family, who indicated originally that Mr Sharif's younger brother would run for the seat, and then the premiership.
But instead the ruling party elected Sharif loyalist Shahid Khaqan Abbasi as prime minister.
It has been suggested he will hold office until elections next year.

John Radford is in a critical condition after being airlifted to hospital following the incident last week.
The 69-year-old is the chairman of Huddersfield and District Cyclists and was promoting a campaign calling for better investigation of road accidents.
Two men have been arrested and bailed in connection with the collision.
The route passed the scene of Mr Radford's accident on the Huddersfield Road in New Mill.
The ride was organised by Mr Radford's two daughters.
His daughter Helen Gorman said the last week had been "horrendous" and that she hoped the ride would raise awareness of cycle safety.
Ms Gorman said: "If this and the campaigning can stop any other family going through what we've been through this week, and what we are going to have to go through over the next few months if dad pulls through, if it helps anyone else then it will be worth it."
The ride raised money for the Yorkshire Air Ambulance.
West Yorkshire Police said a 21-year-old man and a 43-year-old man had been arrested on suspicion of causing injury by dangerous driving.
The pair have been released on bail.
Kulsoom Nawaz, the wife of former Pakistani Prime Minister Nawaz Sharif, has been diagnosed with lymphoma.
[NEXT_CONCEPT]
Hundreds of cyclists took part in a charity ride in Huddersfield to support a cyclist who was left in a coma following a collision with a car.