Port Costa first appeared as a census designated place in the 2000 U.S. census.
!Pop 2010
!style="background-color: #ffffb3;" |Pop 2020
!% 2000
!% 2010
!style="background-color: #ffffb3;" |% 2020
|-
|White alone (NH)
|202
|162
|style='background: #ffffe6; |146
|87.07%
|85.26%
|style='background: #ffffe6; |76.84%
|-
|Black or African American alone (NH)
|0
|2
|style='background: #ffffe6; |0
|0.00%
|1.05%
|style='background: #ffffe6; |0.00%
|-
|Native American or Alaska Native alone (NH)
|3
|2
|style='background: #ffffe6; |2
|1.29%
|1.05%
|style='background: #ffffe6; |1.05%
|-
|Asian alone (NH)
|3
|7
|style='background: #ffffe6; |5
|1.29%
|3.68%
|style='background: #ffffe6; |2.63%
|-
|Native Hawaiian or Pacific Islander alone (NH)
|0
|0
|style='background: #ffffe6; |0
|0.00%
|0.00%
|style='background: #ffffe6; |0.00%
|-
|Other race alone (NH)
|0
|0
|style='background: #ffffe6; |0
|0.00%
|0.00%
|style='background: #ffffe6; |0.00%
|-
|Mixed race or Multiracial (NH)
|8
|7
|style='background: #ffffe6; |19
|3.45%
|3.68%
|style='background: #ffffe6; |10.00%
|-
|Hispanic or Latino (any race)
|16
|10
|style='background: #ffffe6; |18
|6.90%
|5.26%
|style='background: #ffffe6; |9.47%
|-
|Total
|232
|190
|style='background: #ffffe6; |190
|100.00%
|100.00%
|style='background: #ffffe6; |100.00%
|}
The 2020 United States census reported that Port Costa had a population of 190. The population density was . The racial makeup of Port Costa was 81.1% White, 0.0% African American, 1.1% Native American, 2.6% Asian, 0.0% Pacific Islander, 1.1% from other races, and 14.2% from two or more races. Hispanic or Latino of any race were 9.5% of the population.
The census reported that 100% of the population lived in households.
There were 104 households, out of which 9.6% included children under the age of 18, 33.7% were married-couple households, 6.7% were cohabiting couple households, 36.5% had a female householder with no partner present, and 23.1% had a male householder with no partner present. 43.3% of households were one person, and 25.0% were one person aged 65 or older. The average household size was 1.83. There were 50 families (48.1% of all households).
The age distribution was 9.5% under the age of 18, 6.3% aged 18 to 24, 21.6% aged 25 to 44, 37.9% aged 45 to 64, and 24.7% who were 65years of age or older. The median age was 50.5years. For every 100 females, there were 91.9 males.
There were 106 housing units at an average density of , of which 104 (98.1%) were occupied. Of these, 50.0% were owner-occupied, and 50.0% were occupied by renters.
Education
Port Costa is in the John Swett Unified School District.<!--UNI 18990--> It previously operated Port Costa School.
Notable people from Port Costa
- Clayton Bailey and Betty Graveen Bailey