Two of the stowaways were taken to hospital for treatment and the remaining 23 were taken to a "police location".
Police in the Netherlands say 25 stowaways have been found on a ferry bound for the UK.
The stowaways were discovered in a refrigerated container on-board the ship a few kilometres after setting sail on Tuesday evening, emergency coordination organisation Rijnmond Veilig said in a statement.
The Britannia Seaways ferry, run by DFDS Seaways, had left Vulcaanhaven in Rotterdam-Vlaardingen and was heading to Felixstowe in England.
Two of the stowaways were taken to hospital for treatment and the remaining 23 were taken to a "police location", Rijnmond Veilig said on its website.
A number of ambulances arrived at the port and sniffer dogs were being used on the ship to search for any other stowaways.
Dutch broadcaster NOS, citing a police spokesperson, reported that the ferry turned around and headed back to Vlaardingen after the discovery.
The ship's crew reported that no one died, according to Rijnmond Veilig.
"Emergency services are reporting about 26 people," a statement posted on the local government website said.
"Word from the ship is that no one died."
Further details were not immediately available.
The incident comes weeks after 39 bodies, all believed to be Vietnamese migrants, were found in a lorry in Essex on 23 October. Two people have been charged in Britain and eight in Vietnam over the deaths.