Thirteen others were injured and two were reported missing.
Five were killed in flash flood at Ramu, and two died after trees fell on them during the storm at Saint Martin’s Island on Friday.
A child was killed and four were injured in a landslide at Ghonar Parha hill in the city.
Another landslide in Bandarban killed two children.
The Met office forecast that the bad weather might continue for another day, as the south-western monsoon is on.
It recorded 210-millimetre rainfall in Cox’s Bazar from Friday morning to evening.
Besides Cox’s Bazar and Bandarban, several areas in Bangladesh, including Chittagong, Chandpur, Faridpur, and Jhenaidah have been submerged.
In Bandarban, the deceased were identified as 11-year-old Md Alif and his sister eight-year-old Farzana Mim.
Their father, Abdur Rajjak, and another person, Shah Alam, were injured and have been admitted to the Sadar Hospital after the incident on Friday.
“Continuous rainfall over the last six days led to the collapse of a hill on Rajjak’s house around 3am,” Bandarban police OC ‘Amir’ told bdnews24.com, quoting locals.
“The children were sleeping inside the house and died on the spot.”
Police and fire service personnel recovered the bodies with local help.
Thousands have been stranded in the hill district as its road links with other places have been disrupted due to landslides and floods caused by the rain.