Square co-founder Tristan O’Tierney has left the US mobile payments company after more than four years there.

O’Tierney announced his departure on Friday — via Twitter — confirming that he has no immediate plans other than taking some time off to travel.

The spotlight at Square so often tends to shine on charismatic co-founder Jack Dorsey — who also founded Twitter — but O’Tierney played a crucial part in the company and its early development after joining in January 2009.

The former Apple and Yahoo engineer built Square’s original iPhone app — which got a significant revamp this week — and led the development of its first iPad app, the Pay with Square service and its Register app for merchants.

There’s not a great deal of information about O’Tierney’s departure at this point, but it looks to have been on good terms. It is often the case that early-stage employees (and/or founders) get to a point where the once-small company they worked at has become a much bigger organization that no longer feels like a startup, or they feel that they’ve achieved enough to go on and start something else from scratch. That could well be the case here

(Update: yes, O’Tierney says his departure is amicable)

Last year was a huge one for Square which saw it close a $25 million investment and partnership with Starbucks, as part of a larger $200 million Series D round that reportedly valued the firm at $3.25 billion.

As part of the deal, Square payments are accepted at some 7,000 Starbucks chains across the US. As of September 2012, Square was processing more than $8 billion in payments on an annual basis.

Square is building its presence in its first market outside of North America — after it launched in Japan in May — but it has seen a number of high-profile departures this year.

COO Keith Rabois left due to a sexual harassment suit from an employee, while Vice President of Partnerships Alex Petrov and Vice President of International Alyssa Cutright were revealed as gone just days before the Japanese launch.

Cutright, who departed in March, was responsible for the company’s first international expansion into Canada. Petrov’s exit was particularly strange since he was hired from PayPal at the beginning of May, and left without actually starting at Square.

Image via Flickr / Sagolla