Tristan O'Tierney, one of Square's team of co-founders is leaving the mobile payments company after four years.

O'Tierney, who Tweeted about his departure, did not reveal his future plans other than a desire to travel. A former Apple and Yahoo engineer, O'Tierney built Square's iPhone app and was a lead developer for the company's first iPad app, Square Register for in-store payments, and the Pay with Square service.

O'Tierney's the latest in a series of executive departures at Square. Alyssa Cutright, vice president of international and Alex Petrov, vice president of partnerships, both recently left the company; as did Rob von Behren, an engineer who left Square to join Braintree. Square chief operating officer Keith Rabois left the company in January amid a sexual harassment allegation.

Square recently hired Demetrios Marantis, a former Obama administration trade official, to aid its international expansion strategy. It also hired Francoise Brougher, a former Google executive, as business lead.

The personnel changes come as Square aggressively pursues a larger merchant base, via both its Square Register software and the Square Stand card reader, and expands into Canada and Japan.

Square also recently redesigned its Square Wallet mobile payment app


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