The South Korean electronics giant, which had poked fun at rival Apple over reports that its new iPhone model was susceptible to bending, said the "Gapgate" complaints concerning the Galaxy Note 4 were groundless.
After the "phablet" went on sale in China and South Korea on Friday, a number of South Korean users posted or tweeted pictures showing how a business card could be inserted between the display panel and the frame.
Some warned the gap would collect dirt and might even allow fluid into the device.
Samsung insisted the gap was a natural part of design that "does not impact the functionality or quality" of the phone.
"We assure our customers that all Galaxy Note 4 units meet our strict manufacturing and quality control standards," it said in a statement.
A company spokesman added that the model had passed rigorous tests for dirt protection and water resistance.
Samsung initially pioneered the market for the "phablet" devices sized between a smartphone and a tablet computer when it introduced the Galaxy Note series in 2011.
The two rivals have locked horns for years in their battle to dominate the global smartphone market, suing each other for alleged patent violations and using aggressive advertising.
In a recent ad for Galaxy Note, Samsung showed a bent phone bowing to a straight phone with a line "Bend to those who are worthy".