Mr Rubin left Google in 2014 to create Playground, a technology investment company. Essential is one of the companies it funds – and the Essential Phone is its first product.
The device runs the Android operating system and has gone on sale in the US for $699 (£540).
However, it will be difficult to break into the huge market dominance of established brands.
Samsung currently has the largest share of the smartphone market, with 21% of global sales, according to analyst firm Gartner. Apple enjoys 14%.
Aside from Chinese giants offering subsidised devices, smaller firms have found it difficult to command even single-digit market share – and the Essential Phone’s pricing will set it an even greater challenge.
But Mr Rubin told an audience in California that he believed there was space in the Android market for another big player besides Samsung, and that Essential was aiming to be a major consumer brand with several products.
As well as Phone, Essential will “soon” sell Home a standalone assistant similar to Amazon’s Alexa and Google’s Home.
The Essential Phone is made from titanium and has a screen that stretches across most of the device, with minimal bezel.
On the reverse, two magnetic dots are designed to make it simple to snap-on accessories, such as a 360-degree camera add-on the company is also offering.