Components are plugged in by pulling off the base of the G5 phone and swapping it for an alternative unit.
Google is known to be working on a modular phone of its own, but the concept is far from mainstream.
One
expert said the idea would help LG stand out from its rivals, but added
that it was too soon to know whether it would turn around the firm's
fortunes.
The firm also announced new virtual reality kit and a remote control
robot at its Mobile World Congress press conference in Barcelona.
In January,
LG reported that its smartphone shipments had fallen by 2% in 2015 despite the market as a whole growing by about 10%.
Plug-in parts
LG revealed two optional modules - which it calls "friends" - alongside the new mobile handset.
One
is a 32-bit digital to analogue converter (DAC) made by the audio
specialist Bang & Olufsen, which it says makes the phone capable of
playing sounds at a higher resolution - meaning music should be more
detailed when heard via good quality headphones or speakers.
The other module is a camera controller that the company says should
make it easier for owners to take pictures using only one hand. It
features a dial for zooming and a button to take photos among other
controls, and also provides a 40% boost to battery life.
A
spokesman for LG told the BBC its engineers were working on further
modules, and it hoped other third-party manufacturers would also add to
the ecosystem.
Switching between modules involves pulling out the
phone's battery and attaching it to the new part. Although this makes
the action more fiddly, it does have the benefit that the battery can be
easily replaced, unlike in many other handsets.
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