“5% for 5 days.” That is how Plum’s chief executive, Victor Trokoudes, pitches the new Cash ISA offer, and the arithmetic of the headline rate deserves a closer look.
The savings app is paying 5% AER (variable) to new customers who open a Cash ISA between 25 and 30 September 2026. But 2.46% AER of that is a bonus that lasts only 12 months. The catch is what happens after the bonus ends: the rate falls back to the underlying one, and it is variable, so it can change.
What the offer says
Trokoudes said September is when people look carefully at their savings, with summer spending over and the festive season and new year ahead. Plum’s rate, he said, is “far higher than both the Bank of England’s base rate and the inflation rate, ensuring that customer money is growing in real terms while also being protected from tax.”
Adam French, head of consumer finance at Moneyfactscompare.co.uk, called it “comfortably market-leading.” He also warned tax-savvy savers to act fast, since the rate is only for new customers who open through the mobile app inside that window, and that the underlying rate is subject to change.
The small print
Interest is paid monthly and the minimum balance is £1. The account is flexible, so savers can withdraw money and replace it in the same tax year without touching their £20,000 annual Cash ISA allowance. Automated saving rules are available. Savers can also split money across Plum’s Cash ISA, stocks and shares ISA and Lifetime ISA.
The restrictions matter. The 5% rate goes only to customers who have not opened a Plum Cash ISA before. Transfers in are allowed but earn a lower 3.75% AER (variable), including a 1.21% bonus. Transfers out are permitted, but no interest is paid for the month in which a closure or transfer out is requested.
Plum is not a bank. It says it uses “bank-grade protection for your personal information” and that customer money is held with authorised banks.
