She said: "Over 1,057,000 people have been killed by guns in the USA since John Lennon was shot and killed on 8 Dec 1980.
"31,537 people are killed by guns in the USA every year. We are turning this beautiful country into war zone.
"Together, let's bring back America, the green land of peace.
"The death of a loved one is a hollowing experience. After 33 years our son Sean and I still miss him. Yoko Ono Lennon."
Ono and Lennon were married in Gibraltar on March 20, 1969, and went on honeymoon to Amsterdam where they famously campaigned for peace by spending an entire week in bed.
Lennon was murdered outside his New York apartment on December 8, 1980.
The 40-year-old was shot four times in the back by Mark Chapman who had asked for his autograph only hours before he laid in wait and killed him.
The recent calls for tougher gun control laws in the States follows the December 14 mass shooting that killed 20 schoolchildren and six adults in Newtown, Connecticut - and the Aurora theatre massacre in Denver last summer which claimed 12 lives.