Impact of Mergers on Prosperity Bancshares Q1 2026 Market and Margins

Executive takeaway (Q1 2026)

In Q1 2026, Prosperity Bancshares’ recent merger activity materially expanded its scale and changed its gross margin drivers primarily through (1) higher loan and deposit volumes from acquired banks and (2) net interest margin expansion driven by asset repricing and the merger mix, while also creating near-term earnings drag from merger-related costs. ¹²³

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1) How the mergers affected Prosperity’s market presence (Q1 2026)

Scale expansion in loans and deposits

• American Bank Holding merger (completed Jan. 1, 2026) and Southwest Bancshares merger (completed Feb. 1, 2026) both closed during Q1, directly increasing Prosperity’s footprint and balance-sheet presence during the quarter. ¹
• Reported loans rose to $25.2 billion at March 31, 2026, up $3.3 billion (+15.1%) versus $21.9 billion at March 31, 2025. ²
• Management explicitly attributed the loan growth primarily to the mergers with American and Southwest. ²
• Reported deposits rose to $32.6 billion at March 31, 2026, up $4.6 billion (+16.4%) versus $28.0 billion at March 31, 2025, with management attributing deposit growth primarily to the mergers. ²

What this implies for “market presence”

While the excerpts don’t provide branch-count or geography-by-market metrics, the balance-sheet impacts show that, by Q1 2026, Prosperity’s intermediation footprint expanded quickly via acquired loan/deposit bases (i.e., greater market penetration through customer relationships and funding sources). ²

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2) How the mergers affected Q1 2026 gross margin drivers (earnings margin logic)

For banks, “gross margin” is not reported the same way as industrial companies; the closest functional drivers in the excerpts are:

• Net interest margin (NIM) / net interest income, and
• Noninterest income/expense, including merger-related costs and efficiency.

A. NIM and net interest income: mergers + repricing drove margin expansion

Net interest margin (NIM):

• Prosperity reported NIM of 3.51% (tax-equivalent) for Q1 2026, up from 3.30% in Q1 2025 and up from 3.30% in Q4 2025. ³²
• Management stated that net interest income and NIM increases were primarily due to repricing of earning assets and addition of American Bank and Texas Partners banks during the period. ³
• Separately, management noted that NIM was affected by the mergers and also by repricing of assets. ²

Net interest income (NII):

• Net interest income before provision was $321.2 million in Q1 2026, up from $265.4 million in Q1 2025, and up from $275.0 million in Q4 2025. ³
• Management again tied the increase in NII/NIM to repricing plus bank additions from the merger activity. ³

Conclusion on gross margin drivers (Q1): The excerpts support that Q1 2026 margin improvement was not purely “mix from acquisitions”; it was explicitly driven by asset repricing alongside the incremental earning-asset and funding-base contribution from the merged banks. ²³

B. Merger-related expenses: near-term drag on operating “gross margin” analog (expense burden)

• Prosperity incurred merger-related expenses of $42.5 million (or $0.34 per diluted share) for Q1 2026 from the mergers with American and Southwest. ¹
• Noninterest expense in Q1 2026 was $217.3 million, up sharply from $138.7 million in Q4 2025; management attributed the linked-quarter increase primarily to merger-related expenses of $42.5 million plus the addition of American and Texas Partners. ³

Conclusion (expense side): Even as NIM expanded, merger integration costs increased the expense base, which would mechanically reduce any “gross margin” style metric based on operating income before credit costs. ¹³

C. Efficiency ratio deterioration in Q1—consistent with merger integration

• The efficiency ratio was 59.2% for Q1 2026, versus 43.7% in Q4 2025 and 45.7% in Q1 2025. ⁴
• Excluding merger-related expenses, the efficiency ratio was 47.6% for Q1 2026. ⁴

Conclusion (margin driver): Integration costs caused a temporary step-up in cost structure; the “underlying” efficiency excluding merger costs was materially better, consistent with management’s emphasis on margin expansion but also acknowledging near-term expense pressure from mergers. ⁴¹

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3) Credit provision and allowance activity: mergers changed credit-loss dynamics, but provision was absent in Q1

This affects net income quality and thus the earnings “margin” context:

• Reported net charge-offs were $41.3 million in Q1 2026, compared with $5.9 million in Q4 2025, and management described it as the largest amount in the bank’s history. ⁵⁶
• No provision to the allowance for credit losses occurred in Q1 2026, but $91.4 million was added to the allowance via the mergers with American and Texas Partners. ⁵
• Management quantified the allowance increase during Q1 due to mergers: $47 million attributable to the American merger and $43 million to the Southwest merger. ⁶

Why this matters for gross margin drivers: While provision expense was not taken in Q1, charge-offs hit earnings. The allowance “uplift” reflects acquired portfolio risk recognition through the merger accounting process, which can influence future provision/charge-off patterns even if Q1 provision was zero. ⁵⁶

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4) Forward-looking indication of margin behavior post-merger (within the Q1 disclosures)

• Management projected continued margin strength: they said they were “pleased with our margin expansion” and expected projected margin for Q2 to be flat to slightly higher than Q1, while noting Q1 had items like $4 million of loan income from nonaccrual loans that they didn’t expect to recur in Q2. ⁷

This reinforces that, although mergers created mix/repricing effects in Q1, the margin trajectory is intended to be supported going forward rather than purely one-quarter timing noise. ⁷³

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Bottom line

In Q1 2026, Prosperity’s completed mergers immediately increased market presence via substantial loan and deposit growth (loans +$3.3B; deposits +$4.6B), largely driven by American and Southwest. ²

On gross margin drivers, the merger impact showed up in two opposing forces:

1. Positive: NIM/NII expansion, attributed to asset repricing and the addition of merged banks, with Q1 tax-equivalent NIM at 3.51%. ³²
2. Negative: integration costs and operating expense inflation, including $42.5M merger-related expenses driving a lower efficiency ratio (59.2%, or 47.6% excluding merger expenses). ¹⁴

Credit dynamics were also affected: the company recorded large net charge-offs ($41.3M) while reporting no Q1 provision, alongside a merger-driven allowance increase ($91.4M total; $47M American and $43M Southwest). ⁵⁶