It took an interval of eleven years, but ultimately Krugman's work on New Trade Theory (NTT) led quite naturally to what is usually called the "new economic geography" (NEG), which Krugman began to develop in a seminal 1991 paper in the Journal of Political Economy.. In Krugman's own words, the passage from NTT to NEG was "obvious in retrospect; but it certainly took me a while to see it. ... The only good news was that nobody else picked up that $100 bill lying on the sidewalk in the interim." This would become Krugman's most-cited academic paper: by early 2009, it had 857 citations, more than double his second-ranked paper. Krugman called the paper "the love of my life in academic work."

Where in NTT Krugman had developed the "home market effect", that same effect features in NEG. NEG explains agglomeration "as the outcome of the interaction of increasing returns, trade costs and factor price differences." If trade is largely shaped by economies of scale, as Krugman's trade theory argues, then those economic regions with most production will be more profitable and will therefore attract even more production. That is, Krugman's trade theory implies that instead of spreading out evenly around the world, production will tend to concentrate in a few countries, regions, or cities, which will become densely populated but also have higher levels of income.

Krugman has also been influential in the field of international finance. As a graduate student, Krugman visited the Federal Reserve Board, where Stephen Salant and Dale Henderson were completing their discussion paper on speculative attacks in the gold market. Krugman adapted their model for the foreign exchange market. In 1979 he published a paper on currency crises in the Journal of Money, Credit, and Banking showing that fixed exchange rate regimes are unlikely to end smoothly: instead, they end in a sudden speculative attack. Krugman's paper is considered one of the main contributions to the 'first generation' of currency crisis models, and it is his second-most-cited paper (457 citations as of early 2009). The literature on financial speculative attacks is vast; a much smaller literature addresses real-side speculative attacks.

In response to the global financial crisis of 2008, Krugman proposed, in an informal "mimeo" style of publication, an "international finance multiplier", to help explain the unexpected speed with which the global crisis had occurred. He argued that when, "highly leveraged financial institutions [HLIs], which do a lot of cross-border investment [....] lose heavily in one market [...] they find themselves undercapitalized, and have to sell off assets across the board. This drives down prices, putting pressure on the balance sheets of other HLIs, and so on." Such a rapid contagion had hitherto been considered unlikely because of "decoupling" in a globalized economy. He first announced that he was working on such a model on his blog, on Oct 5th, 2008. Within days of its appearance, it was being discussed on some popular economics-oriented blogs. The note was not long after being cited in papers (draft and published) by other economists, even though it had not itself been through ordinary peer review processes.

Krugman has done much to revive discussion of the liquidity trap as a topic in economics. He recommended aggressive fiscal policy to counter Japan's lost decade in the 1990s, arguing that the country was mired in a Keynesian liquidity trap. The debate he started at that time over liquidity traps and what policies best address them continues in the economics literature.

Krugman had argued in The Return of Depression Economics that Japan was in a liquidity trap in the late 1990s, since the central bank could not drop interest rates any lower to escape economic stagnation. The core of Krugman's policy proposal for addressing Japan's liquidity trap was inflation targeting, which, he argued "most nearly approaches the usual goal of modern stabilization policy, which is to provide adequate demand in a clean, unobtrusive way that does not distort the allocation of resources." The proposal appeared first in a web posting on his academic site. This mimeo-draft was soon cited, but was also misread by some as repeating his earlier advice that Japan's best hope was in "turning on the printing presses", as recommended by Milton Friedman, John Makin, and others.

Krugman has since drawn parallels between Japan's 'lost decade' and the late 2000s recession, arguing that expansionary fiscal policy is necessary as the major industrialized economies are mired in a liquidity trap. In response to economists who point out that the Japanese economy recovered despite not pursuing his policy prescriptions, Krugman maintains that it was an export-led boom that pulled Japan out of its economic slump in the late-90s, rather than reforms of the financial system.

Krugman was awarded the Nobel Memorial Prize in Economic Sciences, the sole recipient for 2008. This prize includes an award of about $1.4 million and was given to Krugman for his work associated with New Trade Theory and the New Economic Geography. In the words of the prize committee, "By having integrated economies of scale into explicit general equilibrium models, Paul Krugman has deepened our understanding of the determinants of trade and the location of economic activity." The prize was awarded before the U.S. presidential elections raising speculation that political considerations affected the Nobel committee's decision. Economist Gregory Mankiw disputed this, saying, "I see no evidence that the Nobel committee in economics has been politicized at all."

In the 1990s, besides academic books and textbooks, Krugman increasingly began writing books for a general audience on issues he considered important for public policy. In The Age of Diminished Expectations (1990), he wrote in particular about the increasing US income inequality in the "New Economy" of the 1990s. He attributes the rise in income inequality in part to changes in technology, but principally to a change in political atmosphere which he attributes to Movement Conservatives.