Sony, which has been uncompetitive for more than three years, claims its Xperia Z Ultra as being the slimmest large-screen handset on the market. It comes as investors slash the value of Samsung.
At the start of the week, Samsung stock was down 6.2% - capitalization has fallen by $12.4 billion to $187.8 billion.
The new Sony smartphone accepts sketches or notes written using a standard pencil or metal-tipped pen, in addition to an optional stylus. The Xperia Z Ultra is 6.5mm (0.26in) thick - only slightly deeper than the thinnest device on the market, Huawei's Ascend P6.
Sony's new model is a fraction bigger than Samsung's Galaxy Mega.
Unlike the original Xperia Z, the new phone does not need a flap over its headphone socket to protect it from water damage, addressing complaints the feature was fiddly to use.
It can also be submerged to a deeper limit - 1.5m (4.9ft) in fresh water for up to half an hour.
The device also features:
A 1080p resolution screen with in-built software to upgrade lower definition videos and photos
16 gigabytes of internal storage with support for 64GB microSD cards
An 8 megapixel rear camera
A battery offering up to 11 hours talk time or 120 hours of audio playback - a figure which Sony claims is a record
Those concerned about using such a big device for quick tasks are also offered an optional Bluetooth add-on which can be paired to the handset using NFC (near field communication) to make calls, view text messages or stream music.
The accessory is similar to the HTC's Mini accessory announced in January for its 5in Butterfly handset.
The firm says it intends for the device to challenge Samsung's dominance of the jumbo-sized handset sector.
Up until now, Sony, which was once a household name in consumer electronics, has failed to make a dent against their Korean rival in the smartphone market. But analysts are now tipping they are a serious threat.
In past months, Sony's financial position has improved, due to a series of new products. The launch of the new PlayStation 4 will further help the company to take on Samsung. which does not have a gaming console offering say analysts.
According to a study by consultants Transparency Market Research, Samsung accounted for 70% of the overall "super phone and phablet" market in 2012, due to the popularity of models including the Galaxy S3 and Galaxy Note 2.
Earlier this year, it added the Galaxy S4 and Galaxy Mega - a 6.3-inch-screen handset - to its line-up.
Sony already offers a 5-inch handset of its own, the original Xperia Z, which it unveiled in January.
The Ultra follow-up was unveiled at the Mobile Asia Expo in Shanghai.