Innocent fun or Western cultural pollution?
Shops and other outlets, such as restaurants, are officially banned from marking Valentine's Day, in which lovers traditionally exchange cards and gifts.
All shop displays, such as red roses, have to be removed by the end of Wednesday, the AFP news agency reports.
Meanwhile in India, a hardline Hindu party, Shiv Sena, says it will try to stop people there celebrating Valentine's Day, held on 14 February.
Patrols on duty
A Saudi newspaper reports that The Authority for Enjoining Good and Preventing Evil has given retail outlets there three days to rid themselves of Valentine's Day paraphernalia, AFP says.
Non-Muslim events such as Valentine's Day cannot, officially, be celebrated in Saudi Arabia.
Patrols have been organised to enforce the ban.
And teachers have been instructed to warn pupils not to wear red, the colour associated with Valentine's Day.
However, there are signs of defiance.
"Officially, there's no Valentine here. It is banned. But there are a lot of Valentine items you can choose from," one shopkeeper told the Associated Press news agency.
And AFP says one well-known restaurant in the capital, Riyadh, has a newspaper advertisement for a "very distinguished" dinner, on 14 February.
The advertisement, it says, has pictures of red roses.
'No violence'
In India Valentine's Day is becoming ever more popular, especially among the young.
Last year the Hindu Shiv Sena party took part in violent protests against it, calling it cultural pollution from the West.
This year the group has held demonstrations in Delhi, burning Valentine's cards and labelling 14 February a cultural invasion from the West.
A BBC correspondent in Delhi says police will be watching the group closely.
Shiv Sena activists say their protest this week will not be violent.
But neither will they back down in their opposition to what they see as a Christian holiday wrongly celebrated in India.