Armed al-Shabab militants entered the mall in September 2013 and fired on shoppers, leading to a siege over four days.
About half the shops are opening again after an extensive refurbishment.
The reopening comes a week before President Barack Obama visits Nairobi – a sign, the city’s governor said, that the capital was safe.
“Exactly 22 months ago we had one of the saddest days in Kenyan history,” Governor Evans Kidero said.
“As a nation we cried, we mourned, but Westgate is back.”
Somali Islamist group al-Shabab said it carried out the attack in response to Kenya’s military operations in Somalia.
CCTV footage showing terrified shoppers fleeing the gunmen and cowering behind counters. Many were shot as the attackers walked down the aisles of a supermarket.
All four gunmen are believed to have died during the assault.
Parts of the mall were badly damaged by fire and remained off-limits as journalists toured the building earlier in the week. It is not clear if those sections are reopening.
Since the Westgate siege, al-Shabab has launched a number of high-profile attacks, including one on a university in Garissa, north-east Kenya, in which close to 150 people died in April.
Earlier this week, the State Department issued a travel warning to US citizens that extremists could target a summit in Nairobi in late July, which will be attended by Barack Obama.
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