In September 2013, gunmen entered the mall 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 US 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.”
“Today we are excited because we are back on our feet, and we can convince the world that terrorism is not bringing us down,” said Ben Mulla, one survivor of the siege.
But Sunil Sachdeva, an orthodontist who ran a practice inside the mall with his wife, told the BBC he could not return to work there.
“I didn’t leave the country but going back to Westgate – in my opinion, you know, to me it is a graveyard,” he said. “So many people met their death there.”
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.
It was one of the boldest attempts by the armed militants to target foreign nationals as well as Kenyans.
A promised inquiry never happened and a parliamentary committee criticised what it called the laxity and unresponsiveness of security services.
Some uniformed officers were captured on CCTV footage looting shops as the siege dragged on.
And although there are still many unanswered questions about the exact identities of the attackers, and how they were able to smuggle weapons in, the complex management says security has been dramatically improved.