"Permanent or temporary changes to your skin, such as some tattoos, can... impact heart rate sensor performance," Apple said on a support page on its website.
"The ink, pattern, and saturation of some tattoos can block light from the sensor, making it difficult to get reliable readings."
The Apple Watch uses green LED lights combined with light-sensitive photodiode sensors to detect the amount of blood flowing through the wrist, which can then be used to calculate heart rate.
Other problems that have been reported describe the Watch to lock on tattooed skin.
The problem is related to the sensor technology onboard used to detect light reflected back from the wearer's skin. Several smartwatches and wearable fitness devices use similar sensor technology and have also been reported to struggle in similar cases.