The low-cost network - which is powered by Three - has announced that UK customers will be able to get unlimited calls, texts and data for £18.75 a month as part of the SIM only deal.
The offer is live now and will run until 23.59 on October 22.
In a press release, the company revealed: "SMARTY is on a mission to put the trust back into the mobile network industry by doing things properly - simple, honest, mobile.
"SMARTY was started because there's a better way to do things. Low costs, straightforward plans, transparent pricing and flexible deals. "SMARTY even offer money back for the data their customers don't use."
The Unlimited plan has no data speed restrictions or hidden 'fair use policies' in the UK, meaning users can get all the internet they need without worrying about performance or large bills.
For customers travelling to the EU, they will be subject to a fair use limit of 19GB each month.
SMARTY's discount data plans start from just £6.25 for 1GB (£7.50 for 2GB, £10 for 4GB or £15 for 8GB), all of which include unlimited calls and texts, and with money back for unused data.