McDonald's has apologised after a two-year-old child found a used condom in the main dining area of one of its franchises south of Perth and "sucked on it".

The woman, who wished to remain anonymous, told 10daily's Antoinette Lattouf she visited the fast-food outlet in Warnbro with her husband and daughter around 6pm on Friday night.

She told the outlet she was ordering their food at the counter when her daughter found the condom and put it in her mouth.

The mother said it will take two months for them to be able to test their 2-year-old for STDs.

The mother said the black condom looked used, but she said they couldn't find the packaging anywhere.