"It's great that we are attracting players of Mandy's calibre to the club," manager Kelly Chambers said. "Mandy will add to our young squad."
Centre-back Van den Berg, 26, led her country at the 2015 World Cup in Canada and has been capped 75 times.
She played 13 times for Liverpool this season as they finished fifth in Women's Super League 1.

The party won 37.58% of the vote, giving it a majority in the lower house of 235 out of 460 seats.
Civic Platform, which led Poland's coalition government for the last eight years, won 138 seats or 24.09%.
It is the first time a single party has won enough votes to govern alone since democracy was restored in 1989.
Correspondents say the Eurosceptic Law and Justice (PiS) party, led by Jaroslaw Kaczynski, succeeded in attracting votes from people who feel excluded from the country's economic growth.
It offered higher child care benefits and tax breaks for the less well-off.
Its anti-immigrant stance looks likely to set it at odds with the European Commission over migrant quotas in future.
Mr Kaczynski, 66, was not running as prime minister and instead nominated Beata Szydlo, a relative unknown, as the party's choice for the post.
The 52-year-old miner's daughter is seen as a more moderate face to lead the new government, reports the BBC's Adam Easton in Warsaw.
However, some observers said Mr Kaczynski - the twin brother of Poland's late president Lech - could take on the top job himself in the months to come.
Poland returns to conservative roots
Poland elections: Conservative swing grips media
Outgoing PM Ewa Kopacz, of Civic Platform, has already accepted defeat.
Three other parties also won enough votes to get seats in parliament: a new right-wing party led by rock star Pawel Kukiz with 8.8%; a new pro-business party, Modern Poland, with 7.6%; and the agrarian Polish People's Party with 5.13%.
PiS is close to the country's powerful Roman Catholic Church and has promised increased benefits and tax breaks.
It supports a ban on abortions and in-vitro fertilisation and believes a strong Nato is necessary to offset the perceived threat from Russia.
Civic Platform, for its part, sought closer ties with the EU.

The Procession to Calvary, completed in 1602, will remain on show at Nostell Priory in West Yorkshire.
The painting had been put up for sale for £2.7m. A campaign by The Art Fund and National Trust raised £1.7m.
The National Heritage Memorial Fund, which aims to save key historic items, has now stepped in with the final £1m.
The painting depicts Christ carrying the cross on his way to crucifixion and has hung at Nostell Priory, a stately home near Wakefield, for 200 years.
The priory is the family home of Lord St Oswald, who put it up for sale to pay for the restoration of the estate.
He had said he would put it up for auction if the target was not reached by Christmas.
Members of the public donated £680,000 to the campaign, with almost £510,000 coming from trusts and foundations, while The Art Fund gave a further £500,000.
Art Fund director Dr Stephen Deuchar said: "Considering the economic climate, this has been a hugely challenging campaign and we are enormously grateful to all our members and supporters who have given so generously.
"Working with the National Trust has been a very fruitful experience, pooling our resources to pull out all the stops and save this remarkable painting for Nostell Priory and its visitors."
Dame Jenny Abramsky, chair of the National Heritage Memorial Fund, said:  "The overwhelming public support to help secure this stunning painting has been an inspiration.
"Individual giving combined with ongoing support from government funds such as the National Heritage Memorial Fund will play an increasingly important role in securing our most precious heritage."
The fund's money comes from the Treasury and is intended to be the last resort for saving items of importance to the UK's national heritage.
It has received £10m a year since 2007, but its grant will be halved from this year as a result of government cuts.
Netherlands captain Mandy van den Berg has joined Reading Women from Liverpool on a permanent deal.
[NEXT_CONCEPT]
Poland's conservative Law and Justice party won enough votes in Sunday's parliamentary elections to govern alone, final results show.
[NEXT_CONCEPT]
An Old Master by painter Pieter Brueghel the Younger will remain on public display after a government fund paid £1m to save it for the nation.