The Singapore SG Arrival Card, also known as an online tourist authorization, allows entry to Singapore for American citizens.
US nationals traveling for authorized purposes and less than 90 days to the country do not need a Singapore visa. However, they still need to apply for an Arrival Card.
If you are an American citizen planning to travel to Singapore for longer than 90 days, or for long-term work-related, study-related, and/or residence-related reasons, you will need a visa. You are advised to contact one of the Singaporean consulates in the US (Chicago, Los Angeles, Miami or San Francisco), or the Singaporean embassy in Washington DC, to obtain a Singapore visa.