At 29%, mobile's share is slightly lower in the US than the global average, but growth there continued across all retail categories in the first quarter of the year, including 38% growth for the top quartile of retailers.
These are some of headline findings from the US segment of the Q1 2015 State of Mobile Commerce Report released by Criteo, the digital marketing performance firm.
It forecast that mobile's share of ecommerce transactions will reach 33% in the US by the end of the year, by which time the global average is expected to be 40%.
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Also, with the exception so far of the UK and Germany, smartphones are becoming the dominant mobile device for transactions, even though the devices' conversion rates are lower than those for tablets or desktop PCs.
However, with the recent introduction of the iPhone 6 and other smartphones with still larger screen sizes, the report expected mobile to generate a higher proportion of transactions in future.
Smartphones already account for a full 90% of mobile transactions in Japan but over half (53%) of transactions in the US also come from smartphones.
Jonathan Wolf, chief product officer at Criteo, said that with mobile transactions in the US increasing by 10% in just the last three months, "mobile commerce is growing like a weed".
"Smartphones are now the majority of mobile transactions, and the growth of larger screen sizes and better mobile sites is only going to accelerate this trend," he predicted.
In terms of share of mobile transactions by country during Q1 2015, the report placed Japan top with 51%, followed by South Korea (50%), the UK (43%), Germany (30%), Spain and the US (29% each), the Netherlands (27%), Italy (26%), France (22%) and Brazil (12%).
Data sourced from Criteo; additional content by Warc staff