The city’s Culverhouse Cross site is one of 14 sites to be opened as part of a social distancing trial.
Since the closure due to the coronavirus pandemic, customers have only been able to use click and collect services.
The DIY brand said it has implemented a number of security measures and “is now able to follow best practices”.
In a statement, a spokesperson for the chain said, “As an essential retailer, we have seen other essential retailers support social distancing in their stores and are now able to follow best practices and reopen a small number of our stores as a test, while ensuring the safety of our colleagues and customers.
“We strictly limit the number of customers in the store at any time. Customers may therefore need to queue before entering the store.
“To help remind everyone to respect their personal space, we have two-meter floor markers in all of these stores and Plexiglas screens at checkouts. “
To limit contact between buyers and employees, only contactless cards and payments will be accepted.
Cardiff B&Q saw dozens of impatient buyers queuing outside shortly after it opened on Sunday, April 16.
But the brand reminded customers that they should only visit the necessary items.
“We urge all customers to follow government guidelines for social distancing and to only responsibly shop for what is necessary,” said B&Q.