diff --git a/DESCRIPTION b/DESCRIPTION
index cbc7a1937..7e3899917 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -150,6 +150,7 @@ Collate:
     'redundant_ifelse_linter.R'
     'regex_subset_linter.R'
     'routine_registration_linter.R'
+    'scalar_in_linter.R'
     'semicolon_linter.R'
     'seq_linter.R'
     'settings.R'
diff --git a/NAMESPACE b/NAMESPACE
index 75cba1df0..6c63fae68 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -115,6 +115,7 @@ export(redundant_ifelse_linter)
 export(regex_subset_linter)
 export(routine_registration_linter)
 export(sarif_output)
+export(scalar_in_linter)
 export(semicolon_linter)
 export(semicolon_terminator_linter)
 export(seq_linter)
diff --git a/NEWS.md b/NEWS.md
index f5e678d70..b5459598b 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -39,6 +39,7 @@
 * `library_call_linter()` can detect if all library/require calls are not at the top of your script (#2027 and #2043, @nicholas-masel and @MichaelChirico).
 * `keyword_quote_linter()` for finding unnecessary or discouraged quoting of symbols in assignment, function arguments, or extraction (part of #884, @MichaelChirico). Quoting is unnecessary when the target is a valid R name, e.g. `c("a" = 1)` can be `c(a = 1)`. The same goes to assignment (`"a" <- 1`) and extraction (`x$"a"`). Where quoting is necessary, the linter encourages doing so with backticks (e.g. `` x$`a b` `` instead of `x$"a b"`).
 * `length_levels_linter()` for using the specific function `nlevels()` instead of checking `length(levels(x))` (part of #884, @MichaelChirico).
+* `scalar_in_linter()` for discouraging `%in%` when the right-hand side is a scalar, e.g. `x %in% 1` (part of #884, @MichaelChirico).
 * `if_not_else_linter()` for encouraging `if` statements to be structured as `if (A) x else y` instead of `if (!A) y else x` (part of #884, @MichaelChirico).
 * `commas_linter()` gains an option `allow_trailing` (default `FALSE`) to allow trailing commas while indexing. (#2104, @MEO265)
 
diff --git a/R/scalar_in_linter.R b/R/scalar_in_linter.R
new file mode 100644
index 000000000..ea64db002
--- /dev/null
+++ b/R/scalar_in_linter.R
@@ -0,0 +1,41 @@
+#' Block usage like x %in% "a"
+#'
+#' `vector %in% set` is appropriate for matching a vector to a set, but if
+#'   that set has size 1, `==` is more appropriate. `%chin%` from `data.table`
+#'   is matched as well.
+#'
+#' `scalar %in% vector` is OK, because the alternative (`any(vector == scalar)`)
+#'   is more circuitous & potentially less clear.
+#'
+#' @evalRd rd_tags("scalar_in_linter")
+#' @seealso [linters] for a complete list of linters available in lintr.
+#' @export
+scalar_in_linter <- function() {
+  # TODO(#2085): Extend to include other cases where the RHS is clearly a scalar
+  # NB: all of logical, integer, double, hex, complex are parsed as NUM_CONST
+  xpath <- "
+  //SPECIAL[text() = '%in%' or text() = '%chin%']
+    /following-sibling::expr[NUM_CONST[not(starts-with(text(), 'NA'))] or STR_CONST]
+    /parent::expr
+  "
+
+  return(Linter(function(source_expression) {
+    if (!is_lint_level(source_expression, "expression")) {
+      return(list())
+    }
+
+    xml <- source_expression$xml_parsed_content
+
+    bad_expr <- xml_find_all(xml, xpath)
+    in_op <- xml_find_chr(bad_expr, "string(SPECIAL)")
+    lint_msg <-
+      paste0("Use == to match length-1 scalars, not ", in_op, ". Note that == preserves NA where ", in_op, " does not.")
+
+    xml_nodes_to_lints(
+      bad_expr,
+      source_expression = source_expression,
+      lint_message = lint_msg,
+      type = "warning"
+    )
+  }))
+}
diff --git a/inst/lintr/linters.csv b/inst/lintr/linters.csv
index d49834aba..39521e81a 100644
--- a/inst/lintr/linters.csv
+++ b/inst/lintr/linters.csv
@@ -72,6 +72,7 @@ redundant_equals_linter,best_practices readability efficiency common_mistakes
 redundant_ifelse_linter,best_practices efficiency consistency configurable
 regex_subset_linter,best_practices efficiency
 routine_registration_linter,best_practices efficiency robustness
+scalar_in_linter,readability consistency best_practices efficiency
 semicolon_linter,style readability default configurable
 semicolon_terminator_linter,style readability deprecated configurable
 seq_linter,robustness efficiency consistency best_practices default
diff --git a/man/best_practices_linters.Rd b/man/best_practices_linters.Rd
index 99b27e23f..6016a5f66 100644
--- a/man/best_practices_linters.Rd
+++ b/man/best_practices_linters.Rd
@@ -51,6 +51,7 @@ The following linters are tagged with 'best_practices':
 \item{\code{\link{redundant_ifelse_linter}}}
 \item{\code{\link{regex_subset_linter}}}
 \item{\code{\link{routine_registration_linter}}}
+\item{\code{\link{scalar_in_linter}}}
 \item{\code{\link{seq_linter}}}
 \item{\code{\link{sort_linter}}}
 \item{\code{\link{system_file_linter}}}
diff --git a/man/consistency_linters.Rd b/man/consistency_linters.Rd
index e084d2c6f..536c8b53d 100644
--- a/man/consistency_linters.Rd
+++ b/man/consistency_linters.Rd
@@ -30,6 +30,7 @@ The following linters are tagged with 'consistency':
 \item{\code{\link{paste_linter}}}
 \item{\code{\link{quotes_linter}}}
 \item{\code{\link{redundant_ifelse_linter}}}
+\item{\code{\link{scalar_in_linter}}}
 \item{\code{\link{seq_linter}}}
 \item{\code{\link{system_file_linter}}}
 \item{\code{\link{T_and_F_symbol_linter}}}
diff --git a/man/efficiency_linters.Rd b/man/efficiency_linters.Rd
index f2f27d3dc..cc5931d4b 100644
--- a/man/efficiency_linters.Rd
+++ b/man/efficiency_linters.Rd
@@ -27,6 +27,7 @@ The following linters are tagged with 'efficiency':
 \item{\code{\link{redundant_ifelse_linter}}}
 \item{\code{\link{regex_subset_linter}}}
 \item{\code{\link{routine_registration_linter}}}
+\item{\code{\link{scalar_in_linter}}}
 \item{\code{\link{seq_linter}}}
 \item{\code{\link{sort_linter}}}
 \item{\code{\link{string_boundary_linter}}}
diff --git a/man/linters.Rd b/man/linters.Rd
index 60f7ae87b..4b96359e2 100644
--- a/man/linters.Rd
+++ b/man/linters.Rd
@@ -17,18 +17,18 @@ see also \code{\link[=available_tags]{available_tags()}}.
 \section{Tags}{
 The following tags exist:
 \itemize{
-\item{\link[=best_practices_linters]{best_practices} (52 linters)}
+\item{\link[=best_practices_linters]{best_practices} (53 linters)}
 \item{\link[=common_mistakes_linters]{common_mistakes} (7 linters)}
 \item{\link[=configurable_linters]{configurable} (32 linters)}
-\item{\link[=consistency_linters]{consistency} (21 linters)}
+\item{\link[=consistency_linters]{consistency} (22 linters)}
 \item{\link[=correctness_linters]{correctness} (7 linters)}
 \item{\link[=default_linters]{default} (25 linters)}
 \item{\link[=deprecated_linters]{deprecated} (8 linters)}
-\item{\link[=efficiency_linters]{efficiency} (23 linters)}
+\item{\link[=efficiency_linters]{efficiency} (24 linters)}
 \item{\link[=executing_linters]{executing} (5 linters)}
 \item{\link[=package_development_linters]{package_development} (14 linters)}
 \item{\link[=pkg_testthat_linters]{pkg_testthat} (12 linters)}
-\item{\link[=readability_linters]{readability} (51 linters)}
+\item{\link[=readability_linters]{readability} (52 linters)}
 \item{\link[=robustness_linters]{robustness} (14 linters)}
 \item{\link[=style_linters]{style} (36 linters)}
 }
@@ -104,6 +104,7 @@ The following linters exist:
 \item{\code{\link{redundant_ifelse_linter}} (tags: best_practices, configurable, consistency, efficiency)}
 \item{\code{\link{regex_subset_linter}} (tags: best_practices, efficiency)}
 \item{\code{\link{routine_registration_linter}} (tags: best_practices, efficiency, robustness)}
+\item{\code{\link{scalar_in_linter}} (tags: best_practices, consistency, efficiency, readability)}
 \item{\code{\link{semicolon_linter}} (tags: configurable, default, readability, style)}
 \item{\code{\link{seq_linter}} (tags: best_practices, consistency, default, efficiency, robustness)}
 \item{\code{\link{sort_linter}} (tags: best_practices, efficiency, readability)}
diff --git a/man/readability_linters.Rd b/man/readability_linters.Rd
index ddc74a30b..a3ff8f846 100644
--- a/man/readability_linters.Rd
+++ b/man/readability_linters.Rd
@@ -50,6 +50,7 @@ The following linters are tagged with 'readability':
 \item{\code{\link{pipe_continuation_linter}}}
 \item{\code{\link{quotes_linter}}}
 \item{\code{\link{redundant_equals_linter}}}
+\item{\code{\link{scalar_in_linter}}}
 \item{\code{\link{semicolon_linter}}}
 \item{\code{\link{sort_linter}}}
 \item{\code{\link{spaces_inside_linter}}}
diff --git a/man/scalar_in_linter.Rd b/man/scalar_in_linter.Rd
new file mode 100644
index 000000000..3108ca8c5
--- /dev/null
+++ b/man/scalar_in_linter.Rd
@@ -0,0 +1,23 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/scalar_in_linter.R
+\name{scalar_in_linter}
+\alias{scalar_in_linter}
+\title{Block usage like x \%in\% "a"}
+\usage{
+scalar_in_linter()
+}
+\description{
+\code{vector \%in\% set} is appropriate for matching a vector to a set, but if
+that set has size 1, \code{==} is more appropriate. \verb{\%chin\%} from \code{data.table}
+is matched as well.
+}
+\details{
+\code{scalar \%in\% vector} is OK, because the alternative (\code{any(vector == scalar)})
+is more circuitous & potentially less clear.
+}
+\seealso{
+\link{linters} for a complete list of linters available in lintr.
+}
+\section{Tags}{
+\link[=best_practices_linters]{best_practices}, \link[=consistency_linters]{consistency}, \link[=efficiency_linters]{efficiency}, \link[=readability_linters]{readability}
+}
diff --git a/tests/testthat/test-scalar_in_linter.R b/tests/testthat/test-scalar_in_linter.R
new file mode 100644
index 000000000..215639251
--- /dev/null
+++ b/tests/testthat/test-scalar_in_linter.R
@@ -0,0 +1,37 @@
+test_that("scalar_in_linter skips allowed usages", {
+  linter <- scalar_in_linter()
+
+  expect_lint("x %in% y", NULL, linter)
+  expect_lint("y %in% c('a', 'b')", NULL, linter)
+  expect_lint("c('a', 'b') %chin% x", NULL, linter)
+  expect_lint("z %in% 1:3", NULL, linter)
+  # scalars on LHS are fine (often used as `"col" %in% names(DF)`)
+  expect_lint("3L %in% x", NULL, linter)
+
+  # this should be is.na(x), but it more directly uses the "always TRUE/FALSE, _not_ NA"
+  #   aspect of %in%, so we delegate linting here to equals_na_linter()
+  expect_lint("x %in% NA", NULL, linter)
+  expect_lint("x %in% NA_character_", NULL, linter)
+})
+
+test_that("scalar_in_linter blocks simple disallowed usages", {
+  linter <- scalar_in_linter()
+  lint_in_msg <- rex::rex("Use == to match length-1 scalars, not %in%.")
+  lint_chin_msg <- rex::rex("Use == to match length-1 scalars, not %chin%.")
+
+  expect_lint("x %in% 1", lint_in_msg, linter)
+  expect_lint("x %chin% 'a'", lint_chin_msg, linter)
+})
+
+test_that("multiple lints are generated correctly", {
+  linter <- scalar_in_linter()
+
+  expect_lint(
+    trim_some('{
+      x %in% 1
+      y %chin% "a"
+    }'),
+    list("%in%", "%chin%"),
+    linter
+  )
+})