Visitor numbers at the number one attraction in Leeds - according to TripAdvisor - have plummeted from 301,809 in 2004/5 to 193,254 in 2018/19.
Dr Edward Impey, Director General and Master of the Armouries, said the museum experienced major reductions in funding through the 2000s - peaking in 2010/11 which lead to a drop of 21% in visitor numbers in just one year.
The figures were published as part of a data set by the Government's Department for Digital, Culture, Media & Sport, last updated in May 2019.
However, Dr Impey said following major recent investment by the treasury, the museum is expected to grow visitor numbers significantly year on year.
Dr Impey said: "The Royal Armouries experienced well documented major reductions in funding through the 2000s which in 2010-11 led to drastic changes in our signature live programme as interpretation and education staff were laid off, and a drop in visitor figures of 21% from 2010-11 to 2011-12.
"Since then visitor numbers have held steady at around 220,000 per year as we have stabilised and looked to rebuild our offering.
"We did see a drop in visitor numbers at our Leeds museum last year of approimately 10%, which was challenging, but as a largely indoor attraction we weren't alone in this, with factors including an exceptionally hot summer as well as the World Cup playing a part.
"Following major recent investment by HM Treasury and DCMS, we have now begun an exciting period of phased capital investment and programme renewal which we are confident will allow us to grow visitor numbers significantly year on year.
"We are already seeing a positive impact.
"Visitor numbers for 2019-20 are 17% up on last year, thanks in part to that investment, as well as our popular live programme of events and newly refurbished café (and this August we even have an a-maze-ing outdoor maze to ensure we have something for everyone no matter what the weather!)"
The Royal Armouries group of museums as a whole have seen year on year growth since 2016.
Figures for the Leeds site from March 2019, the latest available figures, also show an increase in visitors in the same month for the past three years.
Dr Impey explained how new events later this year are likely to boost visitor numbers for the Leeds museum in the future.
Dr Impey added: "Looking forward we have ‘Make: Believe’ opening in October, our new display and events programme which looks at arms and armour through the lens of popular culture. The display includes some remarkable objects including two blaster pistols from the original ‘Star Wars’ films, armour from the film ‘Excalibur’ and even a helmet from ‘Monty Python and the Holy Grail’, and builds on our existing collections which feature the ‘Lord of the Rings’ trilogy swords, ‘James Bond’ firearms and even a ‘vampire killing kit’.
"‘Tudor Power and Glory’, an exciting new display and events programme on Henry VIII will be opening in April 2020, and we also look forward to presenting a new WWII display opening in May of next year.
"Our position as TripAdvisor's number one visitor attraction in Leeds gives us confidence that whilst we know have much to do, we are on the right track.”