The company, created by Android co-founder Andy Rubin, released its first phone in 2017 but has been largely quiet since.
Mr Rubin tweeted a short video of the device in several different colours, complete with a camera sensor and what appeared to be a fingerprint scanner on the rear of the small, slim device.
The official Essential Twitter account also posted images of the phone, which is known as Project Gem and said it was “working on a new device to reframe your perspective on mobile”.
“It’s now in early testing with our team outside the lab. Look forward to sharing more in the near future,” the firm added.
Other images posted by Mr Rubin appeared to show it running apps including Google Maps and Uber.
The company has not offered any word on if or when it could be released to the public.
Essential has so far struggled to make a substantial impact in the mobile market – the 2017 Essential Phone was initially praised but did not sell in substantial numbers.
Last year, former Google employee Mr Rubin was also the subject of sexual misconduct allegations, from his time at tech giant, in a report in the New York Times.
It reported Mr Rubin received a $90 million severance package in 2014 after Google concluded the accusations were credible.
Mr Rubin has denied the allegations, previously calling them a “smear campaign”.