At loggerheads with the separatist government in the northeastern region, which plans to hold a Scotland-style independence referendum against Madrid's will, Rajoy has launched a so-called "operation dialogue" in a bid to ease tensions and win over public opinion.
Criticised for inaction during his first term in office as independence fervour mounted in Catalonia, Rajoy promised to strive for a rapprochement when he took power again in November to solve what he has dubbed Spain's most serious problem.
Speaking in the seaside city of Barcelona, Rajoy promised "€4.2 billion in investment in infrastructure, transport and housing between this year and 2020, which represents more than a billion a year."
Lack of investment in the wealthy but indebted region has long been a source of contention, pushing some weary Catalans to come out in support of independence, particularly as they see their tax money sent to Madrid and used to prop up other poorer regions.
The belated arrival of the high-speed train to Barcelona in 2008, 16 years after the first such link to Seville in the south, a lack of free highways, and delays in commuter trains are just some of the issues that have contributed to this weariness.
But polls show that many recently converted independence supporters would happily settle for more autonomy and better funding.
Hoping to lure them back, Rajoy pledged to pour money into the commuter train network, airports, roads and ports.
He also promised to finish by 2020 the Catalan section of a Mediterranean freight railway link due to go through coastal regions all the way down to Algeciras on Spain's southern tip.
In addition, Rajoy said he would address the long-time complaint about regional funding and taxes.
"This matters to me. I want Catalonia to be prosperous in a thriving Spain," he said, asking companies for "help in winning the battle for moderation".
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Received with scepticism
But the regional government headed by Carles Puigdemont has reacted with a strong dose of scepticism to these overtures.
"Promises of investment in Catalonia made by the Spanish government have lost all credibility in the eyes of Catalan society," Puigdemont said in an article penned with his deputy Oriol Junqueras and published on Tuesday in the regional daily El Periodico.
They said the central government's previous investment pledges had only been partially fulfilled.
"The state's investment in Catalonia represents 8.2 percent of total investment in regions," they said, arguing this was "way below" what it should be getting given the wealth it generates and how populous it is.
Catalonia is the region that contributes most to Spain's overall economy, accounting for just under a fifth of the total.
As if emphasising this, Puigdemont was in the United States to establish contacts with think tanks and media outlets and defend his plans to call a referendum in September, just as Rajoy made his announcement.
Madrid has warned repeatedly that such a vote would be illegal and against the constitution -- a stance supported by the judiciary.
But the Catalan government has pressed ahead regardless.
Dominated by a coalition of separatist parties, the regional parliament last week approved a 2017 budget that pointedly includes funding for the referendum.
In doing so Catalonia's leaders risk legal repercussions, as illustrated by the two-year ban on public office imposed on former Catalan president Artur Mas this month for organising an illegal independence referendum in 2014, even though the vote was non-binding.