Genève / Geneva to Interlaken by train

There are no direct trains between Genève and Interlaken.
The simplest and fastest option is to make only 1 x change of train at Bern, but is not available in every hour when setting off from Genève.

Also because there are 1 or 2 x trains per hour from Bern to Interlaken; and the gap between these trains can vary from 30 mins to 90 mins, some of the connections between trains at Bern can be longer than the usual 8 - 10mins.
And when that is the case the SBB journey planner suggests an additional connection at Spiez, in order to enable a traveller to reach Interlaken faster.

Alternative Route for Rail Pass Users:

If you have a Swiss Travel Pass/Saver Day Pass or a Eurail or InterRail Pass that's valid in Switzerland, a much more spectacular option for a Genève to Interlaken journey is:

1: Take the train from Genève to Montreux
2: Take a MOB train from Montreux to Zweisimmen
3: Take a train from Zweisimmen to Spiez
4: Take a train from Spiez to Interlaken
OR
3: Take a train from Zweisimmen to Interlaken

Or if you want to head off on a spectacular day trip from Geneve, follow those steps to Interlaken, but then continue the full length of the 'Golden Pass' Route, by taking a train on to Luzern.
There are hourly fast IR trains which will take you back to Geneve from Luzern.

Online bookings open: up to 60 days ahead of the travel date.

Book early and save: Yes

Limited numbers of Supersaver tickets will be made available on most departures, you may need to search through the connections to find them using the ‘earlier’ and ‘later’ buttons.

In some hours the only option is to make two changes of train at both Bern and Spiez, but those more inconvenient journeys don't tend to be any cheaper on this route.
So try and be flexible with your departure and arrival times and target the cheapest journey you can find on your travel date that only has 1 x change of train in Bern.

Using Saver Day Passes:

If you will be making a day return trip to Interlaken from Geneva, be open to the possibility that Saver Day Passes are likely to be a cheaper option than booking return tickets

Seat reservations:

Seat reservations are optional on Swiss IC trains, so seats won't be assigned when booking online.

You can book reservations online for journeys by these IC trains, AFTER you have booked a ticket, by using the SBB (Swiss national railways) seat reservation service - OR request a reservation when booking at a station ticket office.

Note that seat reservations won't be an option if you take the IR service to Bern and/or the combination of trains on from Bern to Interlaken, which require a connection between trains in Speiz.

Travelling with a rail pass:

Rail pass users can jump on any train, without having to make reservations.