The capitals of Finland and Sweden are well connected by passenger ferry services. The distance between Stockholm and Helsinki is approximately 250 miles or 400km, making it a nice overnight trip. It is very common to visit Helsinki from Stockholm over the weekend. For example, departing Stockholm on Friday evening, arriving in Helsinki Saturday morning, and then departing Helsinki Saturday evening and arriving back in Stockholm on Sunday morning. The same type of trip is common from Helsinki to Stockholm. If you live in Stockholm or Helsinki, this trip is an affordable way to escape for a weekend.
There are three regularly scheduled passenger ferries between Stockholm and Helsinki. Since the distance is relatively far, the ships large, and competition low, there aren't so many ferries going direct between Stockholm and Helsinki (like, for example between Helsinki and Tallinn, or North Germany and Denmark). It is a stretch to call these ships ferries anymore, because they are all capable of holding 2-3 thousand people and include an extensive selection of cabins and onboard entertainment like saunas, dance clubs, and live performances. They are closer to cruise ships than ferries.
Option 1: Tallink-Silja M/S Silja Symphony + Serenade Book Now
|Duration||15:45 actual (+1h timezone)|
|Departures per Day||1|
|Helsinki Port||South Harbor Olympia Terminal|
Tallink-Silja operates a year-round once-daily ferry between Stockholm and Helsinki, stopping briefly in the Åland Islands. They have two similar ships, M/S Silja Symphony and M/S Silja Serenade, which are among the largest operating in the Baltic Sea. The crossing takes 15 hours and 45 minutes.
Option 2: Viking Line M/S Gabriella + Mariella Book Now
|Duration||15:40 actual (+1h timezone)|
|Departures per Day||1|
Viking Line has a service basically the same to that of Tallink-Silja. Departing each evening at 16:30, they briefly stop in Mariehamn before continuing to Helsinki and arriving at 10:10 the next morning. The total travel time is 15 hours 40 minutes, making it the fastest current crossing between Stockholm and Helsinki. The ships on this route, M/S Mariella + Gabriella, are equally large and well-stocked for a nice evening.
Option 3: St. Peter Line M/S Princess Anastasia Book Now
|Duration||16:00 actual (+1h timezone)|
|Departures per Day||1 (per week, roughly, see St. Peter Line's rotating schedule)|
|Helsinki Port||West Harbour (T1)|
It is little known, however St. Peter Line has an almost-once-weekly service between Stockholm and Helsinki. The trip is one hour slower than the others, taking 16 hours total. The ship, M/S Princess Anastasia is quite large (holding around 2500 pasengers), and has a similar set of services onboard like you'll find with Viking Line and Tallink Silja. Fares on this ship can be cheaper though, especially if travelling on the off-peak season.
Unlike some other routes (e.g. Helsinki to Tallinn), there isn't a huge difference in the travel time between ferry operators between Stockholm and Helsinki. With Viking Line and Tallin Silja, you will spend around 15 hours on board, whereas with St. Peter Line you will have an extra hour. These trips are always done overnight, and the small differences between the operators is not very apparent.