Protests near the village of Bilin are held almost weekly as Palestinians argue that the barrier being constructed is taking their land and jeopardising any future Palestinian state.
Protesters threw stones and other objects at Israeli forces during the demonstration, the Israeli military said.
Abdallah Abu Rahmeh, a witness who took part in the protest, said Abu Rahmeh, a distant relative, was talking to soldiers when troops fired the cannisters.
"Basem was in the front line of the protest and soldiers fired tear gas at us and one hit Basem in the chest," he said.
"He was about 40 metres away from the soldiers. There was no stone-throwing when Basem was shot."
Earlier, Jewish settlers shot a teenager, alleging that he had tried to attack residents of the West Bank settlement of Hebron.
Residents said the community's "emergency squad" was dispatched after learning a Palestinian had "infiltrated" the area.
A man in Beit Hagai said: "My friend asked him who he was, and the man drew a knife. My friend took my gun and killed the terrorist."
Zviki Bar Hai, a settlement community leader, said the Palestinian was walking around the community, but did not enter any of the houses, YNet News reported.
Palestinian sources contest the account, arguing that it would have been impossible for the young man to have entered the heavily guarded illegal settlement. They said the body showed signs of having been beaten after death.
There have been no claims of responsibility from Palestinian groups for the incident.
Earlier this month, a Palestinian man killed a 13-year-old boy and wounded a seven-year-old with an axe in Beit Ayin, another Israeli settlement in the West Bank.
George Mitchell, the Middle East envoy of Barack Obama, the US president, is visiting the region in an effort to move forward stalled peace talks.
Palestinian officials told him on Friday that the US must press Israel's new government, headed by Binyamin Netanyahu, to accept the principle of a two-state solution.
Netanyahu's government is dominated by people who favour military action, and have suggested Israeli concessions would allow Palestinians to threaten Israel's security.