Rapid assessment surveys of fouling seaweed populations were conducted at 67 sites between Downeast Maine and Staten Island, New York during August of 2000, 2003, and 2005, plus July–August of 2007. A total of 126 taxa were recorded, including 29 Chlorophyceae, 31 Phaeophyceae, 55 Rhodophyceae, four Cyanophyceae, one Xanthophyceae, and six macroscopic colonial diatoms (Bacillariophyceae). Several species were fast-growing nuisance organisms (e.g., Ulva spp.), while seven were introduced taxa. Four introduced species originated from Asia either directly or secondarily (Codium fragile subsp. fragile, Grateloupia turuturu, Neosiphonia harveyi, and Bonnemaisonia hamifera), two were from Europe (Lomentaria clavellosa and L. orcadensis), and one from the North Pacific (Melanosiphon intestinalis). Neosiphonia harveyi was the most widely distributed introduced taxon, occurring at 48 sites (71.6%), while L. clavellosa and L. orcadensis were only found at 3 sites (4.5%). Repeated observations (i.e., 2000, 2003, and 2007) at three sites in Massachusetts documented a recent rapid expansion of the invasive red alga G. turuturu into the Gulf of Maine through the Cape Cod Canal. The numbers of taxa per site were highest between Maine and Massachusetts (28–42 taxa) and lowest (1–13 taxa) in southern New England and New York, presumably because of increased loading of various pollutants within Long Island Sound and near New York City. The highest mean (± SD) number of taxa per state was recorded in New Hampshire (28.8 ± 8.0) and the lowest in Rhode Island (7.1 ± 3.6). The green and red algae exhibited peak numbers in New Hampshire (i.e. 9.0 ± 3.1 and 11.6 ± 5.1), while brown algae were maximal in New Hampshire (8.2 ± 1.1) and Maine (7.8 ± 3.4) and much lower in Connecticut (1.0 ± 1), Rhode Island (1.1 ± 2.9), and New York (1.2 ± 1.0). Fifty-four species were limited to 1–3 sites (1.5–4.7%), while only 7 occurred at > 50% of the sites. The most ubiquitous seaweeds included three opportunistic green algae (U. intestinalis, U. lactuca, and Blidingia minima), two perennial browns (Fucus vesiculosus and Ascophyllum nodosum), and the introduced Asiatic red alga, N. harveyi. Thirty-six taxa were restricted to individual states (31.3%), with Massachusetts exhibiting the highest number of unique taxa (17 or 14.8%) and Connecticut and New York the lowest (one taxon each or 0.9%). Massachusetts also had the highest number of total and mean shared species (86 and 43.3 ± 10.7, respectively), while Connecticut had the opposite pattern (18 total and 15.0 ± 7.5 shared taxa). Cheney's floristic ratio indicated that seaweeds restricted to the north of the Cape Cod Canal formed a cold-water flora, while those limited to the south of the Canal were somewhat less boreal. Most taxa from each state were annuals and exhibited cosmopolitan local distributional patterns, occurring in both open coastal and estuarine sites.