Observatory director Rami Abdel Rahman said that more executions might have taken place elsewhere in Syria that were not documented.
The Syrian Observatory for Human Rights said that in total 14 people had been executed for alleged adultery or homosexuality in the war-torn country since July, half of them women.
Islamic law views all sex outside marriage as a punishable crime.
Other cases of execution documented by the Observatory include a man accused of adultery who was stoned to death by Al-Nusra Front and other Islamist groups in the town of Saraqeb in Idlib.
The rival Islamic State (IS) jihadist organisation is also accused of executing several women and men for alleged adultery or homosexuality.
Another Islamist group threw a man thought to be gay off a building in the northern province of Aleppo.
Al Qaeda's Syria wing, Nusra Front, shot dead a woman in the northwest of the country after accusing her of adultery, a monitoring group said on Wednesday, saying it showed such execution-style killings were not confined to the militant Islamic State group.
Islamic State, which split from al Qaeda and has seized land in Iraq and Syria, has beheaded and stoned to death Syrian and foreign civilians and combatants for crimes it sees as violating its strict interpretation of Islamic law.
But Nusra Front, which sometimes fights alongside Western-backed insurgents in Syria as well as against them, has also carried out such killings or physical punishments after accusing people of violations such as insulting God or thievery.
Both groups have been targeted by U.S.-led strikes which started last year.
Rami Abdulrahman, director of the Britain-based Syrian Observatory for Human Rights, said he received a video showing the woman being shot in the head outside Idlib city after being charged with adultery. Before she was killed, she begged to see her children but her killer refused, he added.
Photos posted on Twitter said to be of the woman showed her dressed in a black robe, headscarf and a red jacket and crouching on a pavement next to a group of standing men who appeared to be from Nusra Front. One man clothed in black wore a balaclava and held an assault rifle.
Other photos showed her falling to the ground beside a wall sprayed with Nusra Front's name and then her body on the ground.
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The al-Nusra Front, or Jabhat al-Nusra, (JN, Arabic: Front victory for the people of Syria Jabhat an-Nuṣrah li-Ahli ash-Shām, "The Support Front for the People of Al-Sham"), sometimes called Tanzim Qa'edat Al-Jihad fi Bilad Al-Sham or al-Qaeda in Syria, is a branch of al-Qaeda operating in Syria and Lebanon.
The group announced its formation on 23 January 2012, during the Syrian Civil War. Since then, it has been described as both "the most aggressive and successful" and "one of the most effective rebel forces" in Syria. The group has been designated as a terrorist organization by the United Nations, Australia, Canada, New Zealand, Russia, Saudi Arabia, the United Kingdom, the United States and Turkey.