Of the 38 people who were killed in the incident, 33 were on Tui holidays.
Announcing its third quarter results, the firm, which owns Thomson and First Choice Travel, said the attack and subsequent warnings about travel to Tunisia would cost it about £20m this year.
Tui is also worried about the effect of migrants on its Greek business.
Since the attack in Tunisia, the Foreign Office now advises against all but essential travel to the country.
"This is the most tragic event and loss of human life that I have ever had to deal with or my company and we remain deeply shocked in terms of the loss of life, those customers that were injured and all the trauma that our customers had to go through, through these terrible events," Tui Group chief executive Peter Long told the BBC.
"We were 100% focused on making sure that our customers were looked after and that was our number one priority."
He also said that the company was reliant on Foreign Office advice on when to resume offering holidays in Tunisia.
The company said that in the three months to the end of June the attack had caused repatriation and cancellation costs of about €10m ($11m; £7m).
Tui also said it had been hit by the continued economic turmoil in Greece, which made withdrawing cash from banks more difficult.
Mr Long said he thought it was time for other European governments to help Greece deal with migrants arriving at Greek islands, especially from Syria.
"We know that tourism for Greece is so important, their economy is fragile and what we don't want to see [is] a deteriorating demand from our customers going to the Greek islands."
However, Tui said that business had been resilient and that summer bookings were still ahead of last year's level.
The company reported a group profit of €49.4m for the quarter, compared with a loss of €5.6m in the same period last year.
Tourism is one of Greece's most important industries, worth about £20bn a year to the country. More than 650,000 people are employed in the holiday sector and more than 22 million people travel to Greece every year from around the world.
In its third quarter results, Tui said that the economic turmoil of the debt crisis had affected demand for Greek holidays, especially from Germany.  A developing issue around immigration could pose a fresh threat to the sector.
"I worry about the publicity putting people off going to the Greek islands," Mr Long said.
"Therefore I hope the Greeks are able to process [the migrants] and I'm sure other European governments will look at ways of helping them through this difficult situation with so many migrants arriving at the same time."
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Travel operator Tui Group has announced how much its business will be affected by the beach attack in Tunisia in June.