October saw deliveries of 2,589 Leafs, bringing the year-to-date total to 24,411--which exceeds the 2014 total of 22,610, the previous record.
Nissan has sold a total of 66,533 Leafs in the U.S. over almost four years.
The Volt is still the highest-selling plug-in car in the country since its December 2010 launch, with total sales through September of more than 69,000 units.
But as the model winds down, it has been selling at a rate of roughly 2,000 per month for almost two years now.
Advocates and industry analysts will also be watching closely to see if the innovative and technologically advanced BMW i3 can maintain the pace of more than 1,000 sales it has set for each of the last two months.
BMW is considered to be the third of the large global automakers--after Nissan and General Motors--to be serious about electric cars, along with startup Tesla Motors.
But sales of the Tesla Model S are not reported monthly, and are consequently the subject of much debate and speculation.
Possibly due to the effects of September's National Drive Electric Week events, October sales are historically high for plug-in electric cars, as advocacy group Plug-In America noted in a recent blog post.
As well as those three vehicles, the highest-volume cars that plug in sold in the U.S. are the Toyota Prius Plug-In Hybrid and Ford's pair of Energi models, the Fusion mid-sizesedan and C-Max compact five-door hatchback.
Following those models is a raft of lower-volume vehicles, both battery electrics and plug-in hybrids, that sell in volumes of 10 to 200 a month.
Among those, the Honda Accord Plug-In Hybrid delivered 34 units in October, bringing its year-to-date sales to 343.
Since it went on sale almost two years ago, Honda has sold 869 of the Japan-built plug-in model of its well-reviewed Accord Hybrid.
Those include the compliance cars sold largely in California solely to meet that state's zero-emission vehicle sales mandate.
Of those, the Fiat 500e and Chevrolet Spark EV will remain on sale through this year and next, but the Honda Fit EV and Toyota RAV4 EV are reaching their required sales targets and will soon be withdrawn from the market.
The Fit EV logged 23 more sales in October, bringing its yearly-to-date number to 370 and its overall sales to 1,032--rapidly nearing the 1,100 Honda has said it will deliver.
October was also be the first month for which deliveries of the Kia Soul EV are added to the sales roster.
This month will see the Volkswagen e-Golf added to the list as well, bringing the total number of plug-in electric vehicles on sale (at least in California) to 21 by the end of the year.
This article will be updated regularly throughout today and tomorrow as sales results for October come in from the various carmakers.