The incident occurred in Nayi Waha village in Ghotki district when the woman's maternal uncle saw her chatting with her fiance.
He and two other relatives chased the couple - Nazreen and Shahid - and shot them dead, police said.
"It is a most tragic case of honour killing. Normally such killings are carried out if a girl or boy have an affair, elope or get married against the wishes of their families. But in this case families of the boy and girl had got them engaged recently," Daud Bhutto, a senior police official in Ghotki said.
The two suspects, both of whom happen to be the girl's uncles, have been arrested, the police said, adding that further investigations into the matter was underway.
The Human Rights Commission of Pakistan (HRCP) has reported an average of 650 honour killings annually over the past decade. But since most go unreported, the real number is likely to be much higher.
Last year in Pakistan's biggest city, Karachi there were at least three cases of honour killing where tribal elders ordered couples to be killed for insulting and bringing shame to their families.